What has to be true about the slab underneath first
An overlay only works because it bonds to the concrete already there, which means it inherits everything that concrete is doing. A slab that is sound, flat, and not actively cracking or settling is a fair candidate. A slab with a crack that keeps moving, a corner that has dropped, or a base that is washing out underneath it is not, because whatever is happening down there keeps happening right on through the new layer.
That is the same reasoning that rules out pouring a plain topping over a failed driveway, and it holds just as true for a decorative overlay. Surface prep matters as much as the product itself: the existing slab gets profiled or roughened, cleaned of anything that would keep the new layer from gripping, and sometimes primed with a bonding agent before the overlay goes down.
Overlay versus resurfacer
Both go on top of existing concrete, and that is where the similarity ends. An overlay is thicker, often a half inch or more, applied heavy enough to be stamped, textured, or troweled into a real pattern, and it can smooth out light surface flaws in the slab underneath it. A resurfacer is thinner, closer to a skim coat, meant to renew a worn broom finish or a dull surface rather than reshape it into something new. If the goal is a stamped or decorative look, that calls for an overlay. If the goal is just making a tired, sound surface look and feel new again, a resurfacer is usually the smaller job.
What an overlay will not fix
An overlay adds a new face, not new strength. It will not stop a crack that is actively moving, it will not fill a void under the slab, and it will not correct a slope that drains the wrong way. Any of those show up again on top of the overlay, usually within a season, because the layer underneath never stopped doing what it was doing. An overlay is a finish decision for a slab that is otherwise sound, not a repair for one that is not.
That is also why a bonded overlay is a different conversation than tearing out and replacing a driveway. Tear out solves a bad base by removing everything down to the ground and starting over. An overlay changes nothing below the surface it bonds to, which is exactly why it only belongs on a slab where nothing below the surface needs changing.