I just started playing Dungeons and Dragons again. The group I'm playing with has characters that were already at 3rd level. I came in early to the session, so the DM and I could get my character (halfling rogue) finished, and level him up to 3rd level.

We played about 30 minutes, until the party go to a point where they met my character. Basically, him and a group of people (NPCs) come running out of the jungle, being chased by a group of "bad guys" (also NPCs, of course).

Both the existing party, and the party my character is in, join together, and after a fairly long fight, kill all of the bad guys. Unfortunately, during the encounter, all but one of the NPCs in my party die. Since my rogue sees no reason to stay with just one person, he asks to join the party of the other players.

One of them asks what I do/what I am... I had no idea how to answer, but one of the other characters suggested I say "I'm in resource acquisition."

Though, I guess, technically true... The barbarian in the group, who's character is not very bright, pushed it a bit farther, wanting to know if I did spells, or was a fighter, or what.

I didn't really want to say it, but I said I killed things, by sneaking up on them from behind... so he calls me a "backstabber" now, haha.

I think the DM would probably let me retcon it just a bit, if I could come up with a better way of saying what I do, without giving away the fact that I'm a rogue.

Any ideas on how to do so?

Convoliution's answer nails the big picture. I recommend reading it first. I'll get into the specifics of "Okay, but what do I actually say to get the barbarian to stop asking questions?"

It sounds like what the barbarian is asking is "How are you useful to us?" There are multiple ways for a rogue to answer this. I've played a few different kinds of rogues over the years, and here are some that I've used.

"I'm a scout."

Talk about how you're light on your feet and can gather information without being seen. This emphasizes your Stealth and Perception skills, as well as possibly Athletics and/or survival.

"I'm good with knives."

Talk about how you're not particularly strong, but you know all the right places to stab people. If the barbarian is particularly sadistic, you could get into the difference between stabbing someone so that they will scream, and stabbing someone so that they won't scream. (At most tables, it might be best to leave the details of that conversation in summary.)

"I do a lot of little things..."

Talk about how there's a lot more to adventuring that just monsters. Emphasize your rogue skills alongside a bunch of other, more benign proficiencies. Sure, you can steal things and sneak up on people, but you can also cook, or tie a good knot, or draw a map. Also you're a locksmith. There's nothing sinister about that.

And one to avoid: "I'm a trickster who loves a good joke. By the way, is this your mug?"

There's plenty of affable charlatans in real life and in movies who will pick people's pockets when greeting them, then hand their stuff back to them as a joke. If you've seen those shows or those movies, it might be tempting to try to replicate those with a character. D&D (especially 5e with bounded accuracy) makes that approach about as safe as Russian Roulette. At best, you'll get a one-off joke that will soon be forgotten by the rest of the party, and at worst, you'll have PVP right when you're introducing your character.

Roll with it.

There may not be a need to retcon anything. You have the option to let things be as they are, and continue your relationship with the party this way.

But it is important to understand why you do not want these party members to know about your rogue-ish nature.

The concept of "rogue" may not be the main concern here.

To begin with, the idea of "character classes" is a part of the game system, not a part of the game's setting. Adventurers would not necessarily introduce themselves as "a Rogue" or "a Barbarian", but rather as "someone who solves problems with stealth and discretion" or "someone who tackles problems head-on and with ferocity". This seems to more or less be the approach you've taken.

So make these aspects of your character apparent. Imagine meeting a stranger in the real-world; it is unlikely that they'd introduce themselves as "extrovert, a sister, an aspiring musician". But perhaps during your chat you notice she's enthusiastic in the way she makes small talk, she mentions her brother, and she taps her fingers rhythmically on the table as the hums along to the background music. None of those labels came directly into play, but she comes across as someone who meets the criteria for those categories.

But clearly there's something about the rogue archetype that you do not want associated with the party's first impression of you. So...

Channel your inner rogue and be discreet about it.

The barbarian wanted to get a sense of what your role would be in an adventuring party. You conveyed to him that you—put bluntly—stab things in the back. You could also, if you'd like, suggest that you have nimble fingers, know your way around locks, are resourceful, and such.

You don't need to reveal everything about yourself; just say/show your skillset in a way that paints you in a non-negative light. But you are still hiding some details, so...

Earn the party's trust.

Everyone has secrets. People in general usually have details about themselves that they prefer to keep private. This is normal and fine, and if necessary you can show reluctance to delve too deeply into your background, especially since you've just met your new party. But you still want the party to let you join them, and not isolate your character. Try not to be shifty and susicious about it. Make it apparent that you can provide a positive benefit to the party.

As mentioned in convoliution's answer, it's not the class that was important but what kind of rogue you are. Not all rogues are stabby assassins (that's actually a subclass) but some can be treasure hunters, escape artists, etc.... Find out why your character is a rogue and talk about that. Also avoid commonalities. For instance, killing by sneaking up from behind can also be a type of monk, a Gloom Stalker Ranger, or... rather anyone can do that.

Another trick is to answer while not answering. For example:

"What do you do?"

"Well, I'm searching for the Eye of Azmarad right now, so I guess I'm a treasure hunter?" (note I made up the eye as far as I know.)

"But how do you fight?"

Place hand on his shoulder. "I fight good, how about you?"

"Ah, do you use tricky tricks magic?"

"You know, I saw you swing that war hammer around... if you ask me, that's the real magic."

Compliments like that tend to get dumber characters off track and you can redirect with questions about them.

Hope that helps.