Key Signature

In music theory, the key of a piece is the group of pitches, or scale, that forms the basis of a music composition in classical, Western art, and Western pop music.

The group features a tonic note and its corresponding chords, also called a tonic, tonic chord, or root, which provides a subjective sense of arrival and rest. It also has a unique relationship to the other pitches of the same group, their corresponding chords, and pitches and chords outside the group. Notes and chords other than the tonic in a piece create varying degrees of tension, resolved when the tonic note or chord returns.

Learn more here: https://en.wikipedia.org/wiki/Key_(music)