Resonance is the quasi-magical force of the Matrix. It allows for the creation of technomancers, technocritters, sprites, and the resonance realms. It is possible otaku and AIs are also touched by resonance.
The process of being attuned to resonance is called emergence. Emergence did not occur until after Crash 2.0 and the reasons for why and how emergence occurs are unknown.
The true nature of resonance is highly debated; a hermetic mage might say it is an alternate expression of magic. A shaman might believe the Matrix is tied to the spirit world, or to Earth itself in some way; a technomancer who follows the technoshaman stream might agree. Other academics believe resonance is the expression of meta-humanity's evolution toward coping with the changing world. Still others believe resonance has always existed, and it is only now that technology has caught up; they cite rumored cases of telepathy as early signs of technomancy.