A flat is when a note in music is lowered a semi-tone (half step) below what would normally be played. If a flat is written within a bar (rather than in the key signature) that flatening of the note remains for the entire bar. When the flat is in the key signature it applies for the entire song.
The orders of flats written in a key signature is B E A D G C F (Battle Ends And Down Goes Charlse Father)
Note: The same pitch can have different names (see Enharmonic) depending on the Key of the scale (C# equals Db).