To me, time and space are directly connected.
Imagine a chair in an empty hallway.
You can sit in that chair, which locates a certain space in the universe you are in, at that particular time. Nobody else can then occupy that chair in that same space-time. They can be in the same hallway, and share some time in vicinity to you, but only you are in that chair occupying the space-time in question.
That is the meaning of existence as far as I can tell. A sentient being (a collection of atoms collected together and perceiving consciousness) that you define as yourself occupies a certain point of time and space in the universe that is unique to you; and nobody or nothing else can occupy that same point on the grid of space and time.
As far as I can tell, you are relatively free to move around and control the space you occupy, so that is the variable on the grid. Time meanwhile moves in a (relatively) logical, linear pattern that is much more inflexible. You can stay sitting in that chair as long as you like (within your mortal limitations) but time will continue to move and flow around you, unbending and constant.
From the reality in which we seem to be bound and cast, the ability to move through time (the constant) would be equivalent to a 2 dimensional object figuring out how to exist in a 3 dimensional world. It is almost impossible for us in our reality to understand what time travel would really mean. We can theorize and contemplate it, but I doubt we can actually grasp it beyond a very rudimentary level.
As I’ve said before, the closest we could get to “time travel” would be finding away to approach the speed of light, therefore slowing down time. But this would be more like "skipping" time than really traveling through it.
The same can be said about the parallel universe theory. As far as I can tell, and I am not dismissing the concept of their being a multitude of possible universes, but again from my own understanding, you can only occupy one point in space and time, not an infinite amount of possible spaces in an infinite amount of times.
This is my very basic understanding of space and time and how they define our existence. Its far from perfect, but it is what I have worked out so far. At this time…