What is the flat sign?

The flat sign is written using the following symbol:

It would sit on a music stave (staff USA) as follows:

It indicates that you should play the note one back (including the black keys on a piano) from the musical note indicated by either the semibreve (whole note USA), minim (half note USA), crochet (quarter note USA) or all the quavers (eighth note USA).

You can recall this by remembering:

The b (#flat) stands for back  i.e go back one.

As an example, D flat or F flat on a piano would be:

Notice how an F flat is the E key.

If a sharp, flat or natural appears in one bar of music it lasts the whole bar unless told otherwise.