Harmony usually means different parts fitting together in a pleasing or coherent way.

In everyday life

In general use, harmony is a state where people, ideas, or elements coexist peacefully and without conflict. It can describe calm relationships, a balanced design, or an overall sense of agreement and tranquility.

In music

In music, harmony is the sound created when two or more notes are heard at the same time, often forming chords. It supports or enriches the main tune (the melody) and helps create feelings of tension and release in a piece of music.