A “key signature” is defined by Wikipedia as follows:
“In Western musical notation, a key signature is a set of sharp (♯), flat (♭), or rarely, natural (♮) symbols placed on the staff at the beginning of a section of music. The initial key signature in a piece is placed immediately after the clef at the beginning of the first line. If the piece contains a section in a different key, the new key signature is placed at the beginning of that section.”
Examples would be as follows:
Key of C Major: No sharps, no flats
Key of G Major: One sharp — F sharp
Key of D Major: Two sharps — F sharp and C sharp
The number of different key signatures is as many as there notes, so there are 12 different possible key signatures.
C, C#, D, D#, E, F, F#, G, G#, A, A#, B
Oh, and there are also flats… so there are really a lot more signatures! But that’s a little beyond our needs at this point, as we are learning to play the piano, not learn music theory.