The Universe is all of space and time and everything. Currently, all of the universe we could observe is in an extremely small, nearly nothing because it is 138,000,000,000 light years but it can also be 600 billion light years. But it's expanding at 76 km per second, so shouldn't it change? Not really, since our units are expanding with it. The universe is currently thought to be 13,799,000,000 years old, yet the oldest star, HD 140283 is thought to be 15,260,000,000 years old. What's up with that? A universe could be described as an independent segment of possibility; a bubble of space with a defined beginning point (a Big Bang the typical sense) and a set of physical laws, which may or may not line up with the ones humanity observes. It is not to be confused with our universe. The only thing aside from a Vecoverse that can be before the Universe is the Law of Dimensional Cycle.
Most known universes are said to contain an Earth, but in most Multiverse cases there can only exist one Prime Earth where in that concept gives us the idea that reality must exist and branch out from the original reality it started from. This idea however varies depending on it's creator whether it be a God or simply a scientific process such as Big Bang which was triggered by a Protoverse known to be the absolute smallest building block/verse that can ever be (or at least discovered to be the smallest as science and theories are always on-going in research).
However, in a multiverse there are multiple universes - perhaps an immeasurable number - which may or may not contain humans or life at all. In fact, some of them may be so hostile that chemistry or physics as we know them do not function properly.
Travel between universes is possible for extremely advanced civilizations through the use of portals, dark energy, time maneuvering, or other space-warping mechanisms. Additionally, beings like God can traverse through universes easily. Most universes shall eventually "die" as the second law of thermodynamics ensures that energy becomes less and less usable over time. This may not always be the case with alternate physical laws, though.