The whole point of a theory is to explain the world. Well, perhaps that is too hasty. Because the existence of a world is really only part of a particular theory about reality, one that is so good that questioning it seems like madness. Perhaps more fundamentally then the point of a theory is to explain our observations. Because the existence of a world containing us in it is a great explanation for our observations. The job of a theory then become to explain this world, or atleast part of it. Which is how I will describe the job of a theory from now on.
In order to explain the world a theory must do two things.
It must convert ostensive definitions to categorical ones*. And it must provide rules describing how the entities postulated by the theory are related to each other. (usually relations in space are taken for granted, and the explanation is cast in terms of how some configuration of entities at one time gives rise to the configuration at subsequent times). Roughly, an ostensive definition is one that picks something out by describing it as that thing which bears some specific connection to us. For example, particular chair might be defined as “that thing over there which seems chair-like”. Such a definition doesn’t presuppose anything about what the chair really is. It only says that whatever is going on, needs to explain how it seems to me. In a way consistent over time and over numerous senses, that there is a chair over there. So explaining that there is a collection of particles which reflects light, ect is one way of explaining it. But an explanation which makes the claim that I and everyone else who thinks that there is a chair over there is hallucinating could be equally successful.