A piece of music which uses notes from a major scale is said to be in the key of that scale. The following song uses the notes from the F major scale, and is therefore in the key of F major.
Because the F major scale contains a B note, any time a B note occurs in the key of F major, it is played as a Bb. This Bb can be written at the start of the music as the key signature of F major. This is the key signature you learned in Lesson 7 .