It means a really fat person, very overweight. It is often used when talking to a fat girl, not used with guys much. The term comes form what a cow does, graze, or eat. A fat person obviously eats a lot so they could be considered cows and would then need to be put out in the pasture for the rest of their life with other fat things.
ME: that chick on the phone wouldn't tell me her weight but told me she wears a size 15.
Friend: dude, a size 15... she is ready to graze