The brain is the internal organ which controls all the bodily functions of a body, either consciously or unconsciously.
The Ancients' brains (and that of Adria) were much more evolved than current humans, and portions of the brain had maximized synaptic activity, allowing them increased intelligence and other natural abilities, as well as mental abilities such as telekinesis, telepathy and healing. The Priors of the Ori are similarly advanced, although not to the same extent, mainly relying on their staffs for anything more than basic telekinesis. (SG1: "Frozen", "Avalon, Part 2", "Origin", "Prototype")
When a human is evolving toward Ascension, the brain's higher functions increase. If technology is used to do this, it may be at the expense of the lower brain functions required for life, giving a choice between Ascension or death. (SGA: "Tao of Rodney")
A sarcophagus is incapable of healing major brain damage. Repeated uses of the device can also affect the mental stability of the user, which is why the Tok'ra do not employ it. (SG1: "Death Knell", "The Tok'ra, Part 1", "The Tok'ra, Part 2")