The flat symbol before the time signature is called the key signature. The key signature appears at the start of every system (each staff line), and key signatures are highly structured orders of accidentals that create the same relationship of notes between Do, Re, Mi, etc. at a different pitch level. In practice mode, you can alter the key up an down to see the same melody expressed in different keys, but performance mode is now locked into one flat, F major, where the pitch F is now called Do.