The principles of art are the same as the principles of design: balance, rhythm, emphasis, proportion, variety, gradation, etc. These are theories meant to help art feel as though it is complete, that it is whole. To have rhythm in a piece of art simply means that the piece has some kind of repetition to it. This could be a repeating kind of line or a repeating shape to create the feeling of movement towards or away from something.
Emphasis means that the eyes are drawn to a specific part of the piece first, and then the viewer takes in the whole thing. It adds something to the piece. Something spectacular. The whole point of using these principles in the art is to unite the piece. To create something that feels whole, and on a small space too.