- Moon pulls the earth.
- Earth pulls away from moon due to centrifugal force.
- In the center of the earth it pulls the earth with the exact same force as the centrifugal force.
- On the side closer to the moon the gravity is more than centrifugal force.
- So water get's pulled towards the moon or "upwards" from earth's perspective.
- That's high tide.
- On the other side centrifugal force is more than gravity.
- On the other side it's the same thing except gets pulled away from the moon.
So since it's pulled on both sides of the earth water is essentially "lighter" and on the sides it's "heavier" if that makes sense. The water flows from the heavier places to the lighter places like down a small slope due to gravity.