About the Rules of Composition:
"Rule of Thirds" is when your subject is placed along a third of the frame, not directly in the center.
"Fill the Frame" is when your subject fills in the whole frame. No background should be visible.
"Avoid Distractions in the Background" is the last rule of composition. You should always avoid distractions in your background. This may be accomplished by opening up your aperture and making sure your subject is far from the background. It can also be done by changing your angle or location.
*If your background adds information to your concept, and is relevant, then it is OK for it to be visible. As in photojournalism.
The rules of composition assist in making aesthetically pleasing artwork. That being said, once you learn the rules, you can break them. You just need to do it with intention and purpose!