Tides in the oceans on Earth are caused primarily by differences in the gravitational attraction of the Moon at different spots on Earth. Notice in the diagram below that, while points B and D are located at the same distance from the Moon, A is closer and C is further away from it.
Since the gravitational force falls off inversely with the square of the distance, the forces acting on the Earth due to the Moon are different at A, B+D and C. The moon pulls harder on A, and less hard on C than it does on B and D. Thereforefore the ocean bulges at both A and C.
Since the Earth rotates, high tides occur twice per day, once when a location is facing the Moon (A) and once when it is facing away from the Moon (C).
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