Tides are what we see as the rise and fall of the levels of the ocean. The tides are caused by the gravitational pull of the Moon and the sun and the rotation of the Earth. The moon location has the biggest effect on tides. The gravitational pull of the moon causes a high tide on the side of the earth directly below the moon and the opposite side of the earth.
@Emma has explained well. Fun fact: That interaction that causes the tides makes the moon gets further away every day (about 3 centimeters every year). That tidal friction transfers energy to the moon, making its orbit bigger. There is this story written by Italo Calvino in his Cismicomics compilation about this fact… It is a lovely story: http://www.aleastory.co.uk/The-Distance-of-the-Moon.pdf