I understand categories like "Character", "Copyright" and so on, but what exactly is "Meta"?

Meta includes more technical details about the image in question. Stuff like what year the image was created, if it's in small, medium, or really high resolution, and the "genre" of the image, when such a thing is easily applicable. Also things like "alternate version available", for when artists will make clothed, lingerie and nude variations of a single pic