A Flat (b) does the opposite of a sharp. Placed immediately before a note, it lowers the pitch of that note by one semitone.
As with sharps, flats are cancelled by a bar line or by a natural sign.
The use of sharps and flats means that the same note can have two different names. For example, F# is the same note as Gb and G# is also Ab These are referred to as enharmonic notes. The following diagram outlines all of the notes in the first position on the guitar (including both names for the enharmonic notes). The first position consists of the open string notes and the notes on the first four frets.