If something is rhythmic, it's repetitive, with a regular beat or pattern. You might drift off to sleep lulled by the rhythmic noise of rain on the roof.
When you think of rhythmic things, you might immediately imagine a jazz drum, but movement can also be rhythmic. In fact, anything or anyone with rhythm can be described this way, including the rhythmic sound of a horse's hooves against the ground or the rhythmic movements of a master martial artist. The Greek root is rhythmos, "measured flow or movement."