The Concept Of Repetition
Repetition is a design principle because it is so fundamental to how people understand things. You get good at golfing by playing a lot of golf. Conventional wisdom suggests people act on advertising after seeing an ad repeated 3-7 times. Visual designs are tied together and strengthened through repetition. Repetition creates rhythm or and a feeling of organization and organized movement.
In other words Repetition is simply the process of repeating elements throughout a design, or several pieces of design collateral to give a unified look. You can think of it as adding consistency to a design.