What is rhythm in music?

Rhythm, in music, the placement of sounds in time. In its most general sense, rhythm (Greek rhythmos, derived from rhein, “to flow”) is an ordered alternation of contrasting elements. The notion of rhythm also occurs in other arts (e.g., poetry, painting, sculpture, and architecture) as well as in nature (e.g., biological rhythms).

Explanation:

Attempts to define rhythm in music have produced much disagreement, partly because rhythm has often been identified with one or more of its constituent, but not wholly separate, elements, such as accent, metre, and tempo. As in the closely related subjects of verse and metre, opinions differ widely, at least among poets and linguists, on the nature and movement of rhythm. Theories requiring “periodicity” as the sine qua non of rhythm are opposed by theories that include in it even nonrecurrent configurations of movement, as in prose or plainchant.

It's the beat of the music, how you feel it. How you count it. It's sort of like, when you're dancing to music or just listening to it, maybe you tap your fingers along or bop your head. You're moving to the rhythm of the song. It's how the entire piece moves along. I hope thing helped out a bit.