What you speak of is simply Einstein's Special relativity and azarimy is correct.

Mathematically at least.

The speed of light is constant in all inertial frames.

Imagine a person travelling in a car with a beam of light bouncing up and down between a space of say, 5 meters. That person would see the said beam of light simply a straight line. Basic physics tells that distance = speed x time. So for the person in the moving car, he will observe the beam of light complete 1 oscillation at time = 5/c (c being the speed of light)

Now take an observer standing on the side of the road. He sees this moving car, and the bouncing light. He will no longer see the light bouncing up and down, instead he will see some sort of zig-zag pattern,i hope you can imagine what i'm trying to say? So the distance the beam travel in one oscillation from the point of view of the observer is definately > 5 meters. Therefore, again from the observers point of view, time taken > 5/c.

So apparently, time in the frame of a person standing still moves faster then the in the frame of the person in the moving car. We however do not feel this difference in time. Therefore, the person moving faster would have 'done' less things as compared to the person standing still. This applies to ALL speeds. This concept is heavily applied in today's technology and non as prominent as GPS satelites.

Back to the maths, so the faster you travel, the longer the zig-zag becomes, the observer will view your 'time' as even slower. This is what people mean when they speak of time travel. Moving fast enough that you can go out to some random star and return to earth and have age less then the people back home.

Further concepts involved in this are the twin paradox and time dilation.

If you are familiar with more advance works, think about warping spacetime instead. So you bring your destination towards you instead!

Theoriticaly possible, but impossible to get the energy required