Help me understand the concept of "worlds" and "source"

[Timat]

They are other dimensions, we just don't know how big they are. Are there really entire universes? No idea. The Source is the original world - the mirror that all the shards were broken from. It is the dimension that we live on, the dimension where Hydaelyn still resides.

Ok so wait....the game and the heroes (us) take place on the source world, not a reflection or fragment world?

Also a few people have mentioned the "thirteenth world". I may not be remembering the cutscene (or maybe it wasnt stated in the MSQ line) but it seems to be that people are saying this is the world where the voidsent come from. How do we know this?