If you are talking about the universe, it means adding space. Inside the universe, it means increasing the volume of pre-extant space occupied by an object or entitiy. You don't expand the physical universe by expanding into space. The physical universe presumably contains "all" known space, although there could be other interpretations.

I don't understand the universe any better than anyone else, but I do know it can be a serious error to project the scale and mechanisms you are used to seeing onto the cosmically large or inversely the sub-atomic.

A