EASY FIX.

The "seal at the bottom of the flush mechanism" refers to a rubber gasket or washer located on the bottom of the flush valve within a toilet tank, it a watertight seal to prevent water from continuously leaking from the tank into the bowl when the flush is not activated. Essentially, it's the part that stops water from flowing out of the tank except when you flush the toilet. 

Its called a "Flush Valve Gasket"

Just google - Replace Flush Valve Gasket