The multiverse theory of quantum physics posits the existence of parallel universes, an infinite number of ever-growing alternate realities, each existing and evolving concurrently with our own. The theory holds that anything that can happen will happen, if not in this reality, then in another. We're part of a meta-verse in which individual sub-universes are continually being generated, some of which are very different, and some of which are almost identical. For every possible universe there are infinite variations diverging at every choice we make like forks in the road.
SG-1 has had several experiences with alternate realities. The first encounter occurred when Daniel was drawn through the quantum mirror on P3R-233, and the team later used the quantum mirror once again after it had been brought to Earth. An alternate SG-1 team deliberately created a subspace rupture that resulted in a ripple effect bringing multiple alternate SG teams into our reality, and an experiment in a parallel universe with inter-universal bridges accidentally drew Carter into an alternate reality.
The multiverse theory is not to be confused with M-theory, which states that there are a fixed number of alternate dimensions existing in parallel with our own physical reality, dimensions that can be accessed by shifting in and out of phase.
Cross Reference: Alternate Reality, M-Theory
Episode Reference: There But For the Grace of God, Point of View, Ripple Effect, The Road Not Taken