Basically, the ethnically/racially Dark Elven (Dunmer) land of Morrowind, known as Resdayn to its inhabitants, is occupied and rather suppressed by the Tamrielic Empire, a human nation that has grown to annex the whole continent of Tamriel, on which Morrowind is located. However, a prophecy exists that the Dark Elven hero Saint/Lord/King Indoril Nerevar, who once had unified the Dunmer against their ancient enemies the Dwemer, will be reborn again as a being known as variously the Incarnate and the Nerevarine, and restore Morrowind to its former glory and liberate it from oppression. And do a heckload of other things, but mentioning them'd be too much spoilers to bear.