A pattern created by repeating or varying elements, with consideration given to the space between them, and by establishing a sense of movement from one element to another.

How do can you "get rhythm"?
Use these two factors:

Repetition: Repeat visual elements consistently.
Variation: Change the color, size, shape, spacing, position, and visual weight of elements.

This rhythm is pure repetition.

Repetition and variation of position and direction give this ad its rhythm.

The rhythm in this ad came built into the photo. Three similar giraffes with variations of pose and spacing.

This web page uses a uniform arrangement of a variety of elements, each set into a repeating border.