The Ra-tet was a mystical order of five enormously powerful (except for the neutral totem), immortal beings who were "totems" or symbolic manifestations of the Egyptian sun god, Ra. This is the most common tale at least, as Wesley stated that their origins have been shrouded in mystery since the dawn of time.
Two of the totems were light, one was neutral, and two were dark. Together, the five totems created an embodiment of the god Ra and day, or daylight. Each individual represented a stage in Ra's journey across the sky. Each of the totems in the Ra-tet also represented a point along the spectrum of good and evil, with the morning totem being the most benevolent and the evening totem the most malevolent. In spite of this, all of the Ra-tet possessed a certain dislike for actual chaos, with even the malevolent totems working to oppose particularly chaotic and uncontrollable evils such as the Granok Demons.
Preserving the Sun
The Ra-tet had an additional purpose: to protect the sun, rendering it insusceptible to earth-bound incantations and the like. To use magic to affect the sun, one had to retrieve each of the talismans hidden within their bodies, which would require killing all of the Ra-tet.
Despite their immense power, The Beast ultimately killed all five totems and retrieved each talisman. With them, the Beast was able to perform a ritual that blocked out the sun, casting Los Angeles into darkness and threatening the entire world. Fortunately, the spell was (somewhat unexpectedly) tied to the Beast, so when he was finally defeated by Faith Lehane and Angelus, the spell was undone and the sun was restored. It is not clear if the Ra-tet were ever replaced or if other being took up their task.