Water in the sea is pulled by the Moon's gravity. The pull is strongest on the near side of the Earth and weakest on the far side, so the ocean is stretched into two bulges, one facing the Moon and one facing away. As the Earth turns under them, a beach meets a bulge twice a day. The Sun adds a smaller pull, about half as strong.
The Moon and Sun are drawn close and the bulges are made much bigger than real, so you can see them. Real ocean bulges are tiny next to the size of the Earth.
NOAA: Gravity, inertia and the two bulges
NOAA: Frequency of tides, the lunar day
NOAA: Spring and neap tides