The circle of fifths organises the 12 pitches of the chromatic scales into a sequence of perfect fifths. Starting on a C, the next note is G (counting C as 1, G is 5), then D, A, E, B, E, F# (Gb), Db, Ab, Eb, Bb, F (returning to C).
Each key has it’s own key signiture showing which combination of notes are to be played in each key.