A Seer Council is a collection of Eldar who walk the Path of the Seer, Eldar who have developed their psychic abilities. Seer Councils serve to train those who have decided to walk the Path, which is the most treacherous of the Paths, as an untrained psyker is a magnet for Daemons. Besides training Eldar on the Path of the Seer, a Seer Council will usually serve as the governing body of a Craftworld (with Craftworld Biel-tan being a notable exception). They will take visions Seers have received and try to interpret them as best as possible. The amount of times this ends up backfiring on the Eldar varies from vision to vision but can be guaranteed when it does to result in lolz.
On the table top they are a mixture of Farseers and Warlocks although in Dawn of War they were all warlocks, perhaps because having a squad of Farseers in the game would have been too overpowered even for them.