Rhythm in art refers to the way that your eye moves through a picture and can be thought of in a similar way to rhythm in music. Your eye will move through some pictures in a structured flowing way creating a strong regular rhythm and conveying a sense of assurance and stability. In other images your eye may be moved rapidly around in abrupt, dissonant changes of direction and speed creating an irregular rhythm that conveys a sense of movement and the unexpected.
For example in the images below, the pattern of identical shapes on the left that occur at regular intervals has a calming reassuring feeling, your eye can move steadily across the pattern without feeling pulled suddenly in unexpected directions. Whereas as the arrangement of shapes on the right change direction, size and shape pulling your eye in vastly different directions, creating a lively exciting rhythm that conveys movement and energy.