A well-made wine should have structure, and this is something that holds true regardless of price. Just because a wine is cheap doesn’t give it an excuse, and the best cheap wines do just that.
Structure means a wine should have a pleasing aroma and a beginning, a middle, and an end when you taste it. You should be able to taste the tannins (if the wine has any), as well as the acid, and the alcohol will stay in the background. In this, the wine is in balance, and that should be the goal for every winemaker.
A flabby wine doesn’t really have any structure, and one part – usually the fruit – predominates. The wine can taste almost syrupy, or more like grape juice than wine, and everything you taste sort of runs together.
The best analogy is an overweight man, whose muscle tone long ago went wherever muscle tone goes. It’s important to note, though, that – like people – a wine doesn’t have to cut and buffed to do its job. Too much structure can be just as bad as not enough, producing a wine that may be technicially correct but that offers little pleasure when you drink it.