Yes, E+ (Economy Plus) is a seat with extra legroom. It does not come with anything other than that though (no free snack, extra seat width, etc). Because Economy Plus seats are treated by UA as seat assignments in the Economy cabin (it does not change your booking code or reissue the ticket), we tend not to use the word "upgrade" to refer to selecting an E+ seat.

Unlike Delta, UA Gold and above can select any open E+ seat at booking (for free), so it doesn't really look like an "upgrade" in that sense. Silvers can select E+ seats starting at check-in, and there are almost always seats left (although sometimes only middle seats).