Middlemarch is the name of the town where almost every scene of novel takes place. It's a pretty average place, as the "middle" part of the name suggests. It's a fictional town, but one that is supposed to be representative of dozens of other towns like it at this point in English history. It's in the "middle" of England, too—north of London, and pretty much in the "middle" of nowhere… you get the picture.
The second part of the title, "A Study of Provincial Life," sounds almost scientific. Eliot wants to suggest that she's giving the reader an objective, anthropological "study" of the way people really lived in the provinces (in the country, in other words – outside of major cities like London, Edinburgh, and Manchester). But how objective is Eliot? Well, science is certainly a major theme of the novel (see the "Themes" section), but the narrator's "scientific" objectivity comes and goes.