What does the letter after the serial number mean on a US 5 dollar bill?

The letter after the serial number is called the "suffix letter" and is simply used to provide more unique combinations of characters.

Numbering normally starts with 00000001A. The numeric part is incremented until it reaches 999999999A, then it's reset to 00000001 and the suffix is increased to "B". Because there are 99,999,999 possible unique numbers and 26 unique letters, it's possible to have nearly 2.6 billion unique numbers for each Federal Reserve District's bills!

Note: 00000000 is reserved for "specimen" or test notes, so it never appears on circulating bills.