Time travel is the idea of going back in time to the past or forward to the future. We always travel forward, to the future. Time travel to the past is not known to be possible, but it is much used in fiction. The Time Machine (1895) by H. G. Wells was one of the first and most famous stories of time travel. Much later, the American movie "Back to the Future" tells a fictional story about a professor who builds a machine that can take people into the future or back in the past.
The popular British TV program Doctor Who is about an alien who has adventures by time travelling. Another TV show about time travel is the series Charmed. Instead of using machines and science, this one uses magic to go to other times and places.
Some series that are not mainly about time travel have episodes about it, for example Star Trek and Stargate and The Flash.