"Miside" is a portmanteau of "Mita" and "Side," implying a parallel dimension or alternate version. The game’s core horror comes from the realization that you are trapped inside the game with a Mita who is not what she seems. Thus, refers to the specific lore universe where these characters exist.
If you are looking for the "Safe of Life" achievement or ending, the verified code for the safe in the basement is Mita Variants mita miside giddora verified