What does the red seal mean on a 1953 US 5 dollar bill?

The red seal indicates the bill is a United States Note, a form of currency printed from 1862 to about 1970. US Notes were a kind of parallel currency that circulated alongside FRN's until they were discontinued to reduce the overhead of printing multiple types of bills.

For values, please see the question "What is the value of a 1953 US 5 dollar bill with a red seal?" for more information.