What is time travel??
Time is relative, both velocity and mass affect time. Time travel is a theoretically possible if a non material "object" exceeds the speed of light.
According to the theory of Relativity an object with mass can not reach or exceed the speed of light. However according to the math of the theory of Relativity if something goes faster than the speed of light time would go backwards for that " object" or entity.
For light which travels at the speed of light time ceases to exist. Theoritically a photon traveling outside of a gravitational field does not experience time. A photon could travel 100 billion light years outside of gravity in an instance experiencing no passage of time.
For Light or another substance that crosses the singularity boundary of a black hole time stops. Within the intense gravitational field of a black hole time does not exists.
Experiments have proven that clocks are affected by being near high gravity and slow down. Also clocks put on planes are slower the faster the plane flies.
Time travel is mathematically possible but not practical at this time.
Time travel is a journey from the present into the future or past.
We used to think of time as a linear progression from the past to the future. We now know that time is the fourth dimension of four dimensional space time. We also know that time is relative and different observers see other observers experiencing time passing at different rates.
In fact we are all travelling in time. We are all moving into the future direction of the time dimension at or close to the speed of light.
We all observe clocks ticking at the same rate. If someone is travelling at high speed or is in a strong gravitational field, we see their clock ticking slower.
It someone travels away from the Earth at close to the speed of light. On their return, more time will have elapsed than the person has experienced. High speed travel is a means of travelling into the future.
Travelling into the past is theoretically possible, but at present we have no means of knowing whether it is actually possible.