A tsunami is a pressure wave that is created when an incompressible fluid (e.g. water) is displaced. If you are taking a bath, and you hold a finger above the water, and then quickly take it below the water, there will be a very brief and fast wave that will travel throughout the bathtub - this is a (very small) tsunami. Large tsunamis, destructive tsunamis, ordinarily happen as a result of earthquakes or large landslides into the ocean.

When a wave approaches a beach, it sucks water that is landward of it into the wave, before the wave arrives. This is why the surf goes back out between waves. The larger the wave, the more suction, and the farther out the surf goes. If the surf goes out a long way, you know that it's being sucked into a very big wave.