Flab is extra flesh or fat on a person's body. Some people take up jogging in the hopes of reducing their flab.
You might notice some extra flab on your belly after a long, inactive winter — even your dog may be carrying around a little flab. Exercise and eating well are the keys to getting rid of flab, if you're unhappy with it. The adjective flabby came before the noun flab, in the late 17th century as a variation on the word flappy, "softly fleshy" in the 1590s.