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.
Images for kids
Transversal time dilation. The blue dots represent a pulse of light. Each pair of dots with light "bouncing" between them is a clock. For each group of clocks, the other group appears to be ticking more slowly, because the moving clock's light pulse has to travel a larger distance than the stationary clock's light pulse. That is so, even though the clocks are identical and their relative motion is perfectly reciprocal.
Time travel Facts for Kids. Kiddle Encyclopedia.