Sometimes it is necessary to use notes which are not within the key signature of a piece of music. This is when accidentals are used. An accidental is a temporary alteration to the pitch of a note. An accidental may be a sharp, a flat, or a natural. A natural sign is used to cancel a sharp or flat. Like a sharp of flat, the natural affects all notes of that pitch for the rest of the bar in which it occurs, unless another accidental occurs after it.
This example demonstrates accidentals applied to the note G in the treble staff. Notice that Gb is the same note as F# which you already know.