It's not 100% known what's going on yet. The main theory is Wanda (who can control reality in the comics....although the movies haven't really touched on that yet) created an alternate reality where she and Vision (who is obviously dead) can live happily ever after. The reality is so convincing even she forgets she created it.
A similar situation happened in the comics, in a story called "Avengers: Disassembled" and "House Of M".
Anyway, in the comics she goes crazy and damn near ends all of reality.
This show ties into the next Dr. Strange movie "The Multi-Verse Of Madness" which deals with alternate realities and time lines. Wanda is in that movie too, probably helping Strange deal with everything she fukks up during this series.
It's just a guess but this show is probably how they'll introduce mutants, the X-Men, F4, etc. because it opens up alternate realities.