The Source is a fundamental energy that fuels all creation. All things stem from the Source, and all things return to it.
Mechanically speaking the Source could be described as a singularity-- a place where all thought, physics, matter, and energy collapse into a single, infinitely dense point. It is the heart of the multiverse and of all itinerant dimensions, of all the energies and magics that make the universe tick.
Those individuals capable of tapping into the power of the Source use what is called the Power Cosmic. Many are destroyed outright by attempting to use this energy, as it requires willpower of a truly cosmic scope to channel. Those with the capability to channel the Power Cosmic can remake reality according to their wishes, limited only by their strength of will, focus, and physical capability to channel the energy required. In many cases, ideologues act as channels of specific metaphysical intent to empower certain people with extraordinary capabilities without being overpowered with the raw power of the source.
The Source is located in a dimensional singularity at the heart of creation, just past the point where individual universes stop having a unique p-brane frequency and collapse into a single waveform. Due to unique spacetime curvature, the Source Wall exists as both a barrier wrapped around the Source and a barrier wrapped around the multiverse that insulates the Source from mundane reality. It is littered with billions of corpses and starships, the relics of creatures that attempted to traverse the barrier. This forms the appearance of a more tangible wall than the Source might otherwise appear.