Alright, so the covenant are a mix of alien races that are all of the same religion, and the believe that one of their calling is to kill all humans. They look upon the halo rings as sacred temple-like places built by the forerunners (ancients of long ago.). The thing is that the rings were actually built as containment units for the flood, which is a parasitic race that feast on the body of its hosts. So in halo one a human ship is attacked by the covenant and they do a random slipspace jump, which just happens to shoot them out next to the ring. They crash land on the ring, and the covenant follow theym there to destroy the people that desecrated their holy place. Well the mastercheif searches for any survivors and discovers the oracle (thats the little metal ball that floats around.) The flood have been awaked by the presence of humans, and start rampaging everything. so the oracle gets the mastercheif to retrieve the key that will activate the rings, effectively killing all life in the galaxy, therefore not allowing the flood to spread. so at the end of it all the mastercheif destroys the ring to keep it from destroying all life and he flies away as it explodes. he gets back to earth and thats where halo 2 picks up.
