A flat of any note decreases its pitch by a semitone. It is the opposite of a sharp, which increases pitch by a semitone. Flats are indicated in standard music notation by the symbol b.
On a piano, the black keys are flats of each white key to their right, and, likewise, they are sharps of each white key to their left. Thus, for example, the black key to the immediate left of B is B flat and the black key to the immediate left of A is A flat.
All scales have at least one flat except for C major, which has no sharps or flats. The largest number of flats a scale can have is seven, which is for the C flat major scale.