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Having read this question, it makes reference to treason, to which part of it states:

The Congress shall have power to declare the punishment of treason, but no attainder of treason shall work corruption of blood, or forfeiture except during the life of the person attainted.

Unfortunately, the archaic language makes it difficult for me to understand. I presume attainder means detention, but I can't work out what "corruption of blood" or "forfeiture except during the life of the person attainted" means or why it's prohibited.

In English law during the late Medieval and early modern period (from 1321 to 1798), it was possible for Parliament to pass a "Bill Of Attainder". This declared a person guilty of a crime, often treason, by legislative act, without any trial or other legal process. See the Wikipedia article for more detail.

Often a Bill of Attainder not only decreed that a person (or people) was guilty, but also confiscated the convicted person's property, preventing his (or rarely her) heirs from inheriting, and possibly rendering those heirs ineligible to hold public offices or peerages. This was called "corruption of the blood", and was viewed with particular horror by many during the colonial period and before.

Attainder was usually followed by execution, possibly by torture. The bill might specify the specific fate of the person attained.

The US constitutional provision prohibits declaring people guilty of crimes by legislative act, and prohibits sentences for crimes that take property or rights from the family or heirs of the convicted person, even in cases of treason. At least that is how it has been interpreted. Fines may be levied, but may only fall on the actual property of the person convicted.

Attainder refers to a metaphorical "stain" (taint, related to 'tint'). In English law, a person could lose their right to pass property to their heirs (especially as punishment for treason). "Corruption of blood" refers to the fact that not only can the children not inherit from the "tainted" person, then cannot inherit from other relatives through the attainted person. That is, the sins of the father will not be legally visited on the children. A person can be punished during his lifetime, but punishment ceases with his death.