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. Then, if we were to push on one piston, the pressure in the water would increase, like before, so the water would be pushing on the other piston, too. If there weren't anything holding the other piston down, then the water's pushing on it would cause it to slide through the tube away from the water.
Pressure is measured as the force felt by a certain amount of surface area.
This means that the water is pushing on the container with a force of 1 pound on every square centimeter of the container. The reason that we care about this is because we can use it with Pascal's Law. For example, let's say we have a container with a big open space in the middle, and two tubes sticking out of it. Like before, there's a piston at the end of each tube. The difference here, though, is that one of the tubes is a lot bigger than the other one.