Matching actions to an external rhythm that one percieves. One of the most basic forms of synchronization is tapping a finger for every beat of a rhythm. Since it is simple, finger-tapping has seen significant use in experiments that study the neural basis of rhythmic processing (Thaut 2008, p. 41).
One theory of the origin of music proposes that the first forms of music were used to coordinate groups of people in large-scale actions such as a hunt, based on the rhythms of the music. This is a more complex form of synchronization. Even today, some cultures use music to synchronize actions such ad raising fishing nets (Thompson 2009, p. 28).