When a song consists of notes from a particular scale, it is said to be written in the key which has the same name as that scale. For example, if a song contains notes from the G major scale, it is said to be in the key of G major.
Instead of writing a sharp sign before every F # note, it is easier to write just one sharp sign after the treble clef. This means that all F notes on the staff are played as F#, even though there is no sharp sign placed before the note. This is called a Key Signature. Key signatures are discussed in more detail in Lesson 6.
Play all F notes as F# as indicated by the key signature.