Imagine you are sitting right at the top of a long slide. At the beginning you are not moving at all. At the moment you let go, it starts: At first you only slide slowly, but the further down you come, the faster you get. You can really feel how the airstream gets stronger and your stomach tingles a little. This happens because your speed increases with every second, you cover more and more distance in the same time.

In physics this is called acceleration: Your speed increases during acceleration or decreases during braking per passing second. On Earth gravity ensures that this happens by itself when falling or sliding. As long as hardly anything slows you down, you get a bit faster with every second; the longer you slide or fall, the greater your speed becomes. That is exactly what acceleration means: that you get faster and faster per unit of time, and when braking you become slower and slower per unit of time.