In music, we calla circle of fifths to an ascending or descending succession of musical notes separated by intervals of fifths, that, in the tempered system, form a closed circle, in which one finds the twelve degrees of the chromatic scale and the order of key accidents that determine the major and minor classical scales. It was first described 1711 by German musician Johann David Heinichen in his treatiseNewly invented and thorough instructions ... for perfect learning of the general bass.
L’harmony which is to the right of another fulfills the function of dominant, and when it is to the left it is a subdominant, leaving the middle one as a tonic. Each tone we reach through the circle of fifths represents a tonal center (C major or minor, G major or minor, etc.). The tonic of each tonality attracts the piece of music to itself, hence the usual reference to a main tonality.
Here I leave you a video that explains very well what the circle of fifths is.