Time is a measurement of movement.
That movement is internal, external, and all across the universe.
We use a standard of time here on Earth, but that may not be the real way to measure time.
But it could be just the difference between units of measurements.
Like the use of pounds or kg for weight, the numbers are different but it is the same weight.
A second is our base of time, but the second may not be the natural function of time, it could be based on some other standard.
Without Time there would be nothing.
What time means can be subjective or objective.
Whether Time is also a dimension is not really a known.
I don't believe in real time travel, so time travel if it exists is just differences in how to measure time. But to actually travel in our time, would mean that the state of every moving thing atoms, subatomic particles would all have to be in the same state. Things that have disappeared would have to exist. How can the universe record and play them back.
Time travel would only make sense if now is not now but either behind the real now, or in front of the real now. Then like a DVR you can move back and forth while now is moving forward.
The universe is already too complicated, and this would be a nightmare, and it wouldn't even be useful.