Dreamweavers were Volsinii who stayed for long periods within the Virtual reality pods who were capable of manipulating their virtual environment. This ability gave them their name by the other 'residents' of the virtual world. They are both respected as well as feared for this ability. They possessed a deep understanding of Human motivations and dreams as well as masters at bringing these to life in their virtual simulations.
After their encounter with SG-1, several Dreamweavers chose to remain inside the virtual reality where they would create scenariors, simulations and assist the Gamekeepers willful nature. A properly prepared Dreamweaver could alter a virtual scenario or simulation at a whim allowing them to produce equipment, weapons and other devices. These changes were easier to draw upon from the experiences from someone who had been hooked within the VR pods. (RPG: "Friends and Foes: Stargate Season Two")