The Seers are humans advanced by drugs an gene technology and they have a high levels of CPA, (Cognitive Predictive Ability). This allows the Seer's to predict the future and other possible outcomes.
he Seers were originally kidnapped from their planets by a mega corporation called Electus. Electus forced the seers into an experiment where they were drugged with Shadow and put into pods that linked their minds together. The reasoning was to test if a collective of human consciousness's could solve problems better than computers.
Data was gathered and analyzed from their collective consciousness, and this revealed that their collective was able to predict events before they happened.
An uprising against Electus made the power com into the hands of a small group of Seers.
Related: Stargate Atlantis?