The moon's gravitational pull creates tides on Earth. As the moon orbits the Earth, its gravitational force exerts a pull on the oceans, causing them to bulge out slightly on the side of the Earth facing the moon. This creates high tide. On the opposite side of the Earth, the water also bulges out slightly, but away from the moon. This also creates high tide.

Between the two high tides, the water level drops to create low tide.

The moon's gravity is not the only thing that affects tides. The sun's gravity also creates tides, but its effect is smaller than the moon's. The shape of the coastline and the depth of the ocean also affect the height and timing of tides.