This sounds rather overwhelming at first, so let's break it down a bit. Pressure is basically a fancy word for how much something pushes on its container and on things in it. For example, air pressure is how hard air pushes on things. When you pump more and more air into your bike tire, you're increasing its pressure. If you increase its pressure too much, then it will be pushing out more than the plastic is capable of pushing in, and your tire will explode.
Water pressure works the same way.
So lets say you have a long, closed tube of water with a piston at one end. If you push on the piston, then you're changing the amount of space that the water has to take up, so it will push back on the piston even more. This means that you've changed it's pressure. By Pascal's Law, we know that not only did the pressure change right next to the piston, but it changed through the whole tube. So now, the water is pushing out on the entire tube more, and it's pushing out in every direction.