The BTFF Multiverse is the set of universes, dimensions, and timelines where we live. It is commonly known as the totality of everything that exists, containing the galaxies, planets and suns.
The BTFF Multiverse is vast, holding many different universes. Within these universes are the different timelines and dimensions. So, Ben Tennyson from Dimension 23 in Universe 4 is different than Ben Tennyson from Dimension 23 in Universe 8.
Commonly, these universes are referred to as Earth-#.