Body is hard to describe. The overall feel of the wine- the bigness- strength (not abv). —another tough one is ‘mouthfeel’. I take it as not the nose(deep aroma) , not the finish, Somewhere between 1st and 2nd base.
When tasting, try to think of the body as the combination of senses: power and intensity of the taste, the heaviness of the flavors and components. Bigger bodied wines, generally speaking, are darker, teeth stainers with powerful tastes and aromas that penetrate your nose while the wine is in your mouth, the senses of taste and smell.
The other part of body is mouthfeel: think of mouthfeel more in terms of your sense of touch, texture or viscosity, thickness, how the wine coats your tongue and the surfaces of the inside of the mouth, hence, how it feels in your mouth, as opposed to how it tastes in your mouth.
All of those senses flood into your brain at the same time, and are affected by all we do as winemakers, extraction, tannin structure, astringency, acidity, ABV, etc. all play a part in the game. My simpleton brain can’t evaluate all that info, but knows what it likes, not so hot at picking out all of the little background flavors, and usually requires several sips / bottles. I’ll never think: “stunning nose of acacia flowers, blackberries, licorice, forest floor, mocha/chocolate, and gobs of cassis fruit”, but more along the lines of “that’s some good sh*t”.