I heard that if you travel at the speed of light or faster (say, to the Kelper systems ) and come back, thousands of years may have passed but you haven't experienced more than a few. How is this possible?
I think you've misunderstood something; there's no generally accepted model for light-speed-or-faster travel, and so there's no generally accepted model for the passage of time in such a mode.
There is a well-known time dilation effect for speeds near, but below, the speed of light ("relativistic speeds"); a complete explanation is both very long and beyond my ability to explain, but the Wikipedia page on Einstein's special relativity might be a good start. The strange apparent behavior of time is a consequence of the fact that the relative speed of light appears the same for any observer which is not accelerating.
Most people think of velocity from a newtonian perspective: if i travel forward at 10m/s w.r.t. the ground, and i another person traveling in the same direction as me at 4m/s w.r.t. the ground, then i will observe that other person traveling at 6m/s.
Light does not work this way. If i’m standing still at 0m/s (let’s say in a vacuum) i will observe light traveling at a speed of “c” m/s w.r.t. the ground. If i’m traveling at 10,000m/s and measure the speed of a particle of light traveling outside of my space ship, i will still measure a speed of c, not c-10,000.
The speed of light (in a vacuum) is constant, no matter what frame of reference you measure it from. How is this possible? Because it just so happens that the passage of time is not constant. This is a fundamental truth about the universe, but it’s very counterintuitive to our normal experience of life. As it turns out, anything that moves with respect to another object experiences time dilation, but it is so small that we just don’t notice it. It’s not until we are flying extremely fast speeds e.g. orbital speeds over a significant period) that it really matters to things like navigation. Precision timing is a very serious thing in space and accounting for time dilation is crucial to operating anything in space,