Time dilation is the difference in elapsed time by two different clocks. In special relativity, this is due to a difference between them in velocity. In general relativity, this is due to a difference in distance from a gravitational well.
Observers will measure that moving clocks relative to him tick slower than a clock that is stationary in his reference frame. This is known as special relativistic time dilation. The greater the relative velocity, the greater the dilation. As one approaches the speed of light, the rate of time reaches zero. This is why massless particles that travel at the speed of light are unaffected by the passage of time.
Theoretically, special relativistic time dilation makes it possible for one to advance into the future in a sufficiently fast vehicle or craft. However, in the real world, this is easier said than done. For example, 6 months on ISS (International Space Station), would cause an astronaut to age 0.007 seconds less than on Earth. Keep in mind, the ISS orbits the Earth at around 7,700 m/s.