You can think of them pretty much like holes. Imagine a piece of paper with a hole in it. Is the hole a real thing? No, it's just the fact that the paper envelops a region where there is no paper. However, in certain situations, it's more meaningful to talk about the hole than the paper, for example when you are trying to fit it in a 3 ring binder. The same is true about electron holes: they're not real, but sometimes it's better to use that description.