Doing my best,
The concept is actually not that complicated once you understand it although the mathematics involved can be quite complicated.
What Einstein discovered was that what we call “time” is actually another dimension in the universe in the same way the 3 spatial dimensions are dimensions. This 4 dimensional fabric is called “spacetime”. Everything “moves” through this fabric at a constant speed of 186,000 miles per second. This speed is a constant.
When you aren’t moving in any of the spatial dimensions all of your speed is through the time dimension. If you move through the space dimension you lose some of your speed through the time dimension. Practically this means that time passes slower for you. This is similar to the way that a car going northeast goes slower in the northern direction then a car going straight north. Since the speed of light is so fast when we move through the spatial dimensions the affects are not noticible. If however you managed to move through space at close to the speed of light the fact that you are passing time much slower then someone standing still who is just speeding through time will become very apparent.