All major scales have the same pattern of tones and semitones, i.e., the interval between the 3rd and 4th note and the 7th to 8th note in the scale is a semitone (1 fret). For the F major scale to keep this pattern of tones and semitones a B flat (Bb) note must be used instead of a B note.
Songs in the key of F major use notes from the F major scale, so songs in the key of F major will contain B flat (Bb) notes rather than B.
The F major scale contains one flat Bb, therefore the key signature for the key of F major contains one flat, Bb.
The most common chords in the key of F major are:
F Gm Am Bb C7 Dm