To restrict is to limit. If you want to eat more healthily, you might restrict, or limit, yourself to eating only foods containing fruits and vegetables –- like carrot cake and apple pie.
You'll often encounter different words related to restrict. A restriction is a law or rule, like the restriction against driving a car without a license. Something that is not permitted is restricted, and this word is often used to designate physical locations that are off limits to the public. You'll often see characters in movies break into restricted areas to obtain secret information.