The Fade is a precarious and enigmatic place, since it is from where demons arise. Throughout the Dragon Age games, the Fade has always been portrayed as a dangerous place, feared by all. In Dragon Age: Origins, if you play the mage origin, one of the first things you do is to take your Harrowing, a trial all mages must take to complete their training. Those who fail are killed by the templars who guard the mages, and those who opt to not take the Harrowing are made Tranquil instead.
The direness of this choice is due to the source of a mage's powers: the Fade. The Fade is a separate realm where the only limitation is one's own imagination, but it is also where spirits and demons reside. A mage's connection to the Fade also exposes them to demonic possession, where they can become an abomination. In Thedas, only mages can be possessed by demons, with the untrained the most susceptible. In fact, in both Dragon Age: Origins and Dragon Age II, you have to travel into the Fade on a quest to save or defeat someone possessed by a demon. In this game, your team enters physically after opening up a rift into the Fade to save yourself, but no one is possessed.
Despite how often the Fade is invoked, there's very little concrete knowledge regarding it. The Chantry warns of the corruption brought by humans entering the Fade, in their foolish attempt to reach The Golden City, where the Maker sits. This in turn brought the Blights, where the dead attack in a large horde—and led by an archdemon—once in a long while to wreck havoc upon the world (this is the central plot to Dragon Age: Origins). A few characters from past games have shared bodies with spirits, possessed but uncorrupted, with varying outcomes. We learn nothing truly good can came from the Fade.
But how true is this? Solas comes to the Inquisition to offer his aid and his knowledge on the Fade, and he is the first character to offer some in-depth understanding of the Fade, outside of the Chantry's teachings. As he is self-taught, he holds none of the Chantry's limited views. The Fade is a reflection of the physical world, and to explore the Fade, one must travel physically, or else be limited to their immediate surroundings. Areas in the Fade are warped by the spirits in the area, who are warped by the history and memory of a location. For instance, spirits gather where emotions are strong, such as a battlefield, since the Veil (that separate the Fade) is thin where there has been violence.
"Your Dalish say that demons hate the natural world and seek to bring their chaos and destruction to the living. But such simplistic labels misconstrue their motivations and, in so doing, do all a great disservice. Spirits wish to join the living, and a demon is that wish gone wrong."
Solas is one of the few dreamer mages (those who can enter the Fade at will, with no extra magical aid or ritual) known in the Dragon Age world. While the Chantry fears such mages (they're known in Tevinter and are usually associated with mages who push their magic too far), the Dalish revere this ability as a part of their culture and heritage, though they hold some overly romanticized views of it. Regardless, entering the Fade through dreams is extremely dangerous, since it attracts demons. Only very skilled mages dare attempt it. Solas admits to regularly dreaming and exploring the Fade and has also interacted with spirits with measured caution.
In Dragon Age: Inquisition, your many companions also act as a depository of knowledge in their particular areas (for example: Dorian on Tevinter, Cassandra on templars, and Vivienne on Orlesian politics). Solas is your one source of knowledge on the Fade, demons, and spirits (as well as elven history, but that's for another topic). After your first arrival at Haven, he can be prompted for various topics, with your dialogue choices determining his approval or disapproval.
"You've got an odd style, Solas. Your spells are a bit different from the Circle mages or the [Tevinter mages]. I've seen self-taught warriors. Even the good ones have something awkward in their style, something that clunks. I don't get that from you."
His intimate understanding of the Fade, on the surface, can be viewed as a video game tool, since often players are thrust into a new world and have to explore by talking to other characters. Solas's knowledge on the Fade can be viewed in such a way, but you may pick up on his depiction as possibly dubious through other characters' observations. These suspicions are confirmed upon the game's epilogue, and with the full awareness on Solas's story, his knowledge of the Fade makes total sense in the story, and are only a sliver of what he really knows.