The tides are the result of the moon exerting its gravitational force on the ocean and bulging it both toward and away from the moon. The tide is higher, the ocean is higher, at the location closest to the moon and on the opposite side of the Earth.
As the Earth rotates, the position relative to the moon changes, so the bulge moves. At any one location the sea level goes up and goes down as the bulge passes that location. Every six hours the tide goes from high to low and high to low again. Over 24 hours you typically see two high tides and two low tides.
