A guy called Kurt Godel (who is worth looking up if you get the chance) solved Einstein’s general relativity equations in a rotating universe. The solution said the universe should have closed time like curves, which basically means time travel would be possible in such a universe.
Chris and Jack answered this pretty well, but there’s a tiny bit more to say.
In our Universe, given its geometry, time travel might be feasible, but it would be very difficult to achieve.
Essentially, the only way to time-travel would be for time to curve back on itself. The one plausible way to do this might be through a worm-hole, where two black holes join end-to-end. That’s extremely unlikely to happen, and would require the geometry of time in our Universe to be a bit weird (it’s probably not like this, but there’s a small chance it might be). Something like this:
Obviously, the capacity for time travel would be very limited, and it would only go between two fixed points in space and time, even if it were viable.