What does the red stamp on a two dollar bill mean?

Answer 1

My best guess at what you mean by "red stamp" is that you have a bill where the treasury seal is printed in red. If what you're describing is a circular image with a picture of a shield in the middle, and on the shield is a balance at the top and a key at the bottom with a chevron containing stars between, this is what it is. The red color (probably) indicates that it is a United States Note, which is a particular type of banknote issued from 1862 to 1971. (Other kinds of banknotes printed overlapping with the same period had green, blue, orange or brown seals, with the color indicating what kind of note it was).

In practice it doesn't really matter except to a collector. United States Notes, though no longer printed, are still valid currency, though they're less common than Federal Reserve Notes (with a green seal and still being printed) and might therefore be worth a small premium over face value to a collector.

If you mean something else, it's hard to say without more details.