In music notation, a flat is a symbol resembling a lower-case 'b' which indicates that a certain note should be lowered a half step. If used in the key signature, it means that note should be lowered a half-step whenever it is encountered; if used as an accidental, it means that only that note should be changed.
The half-step decrement applies whether it changes the note to a "white key" or not. For instance, A-flat is the same as G-sharp, but F-flat is the same as E.
A flat looks something like this, when used in the key signature and as an accidental with a half note:
/\
----| /----------------------------------------------------
|/ b
----/--------------b O-------------------------------------
/| |
--/-|/\--b----------|--------------------------------------
| | | b |
-|--|--|------------|--------------------------------------
\ | |
---\|_/----------------------------------------------------
|
\|