As the Moon's gravity tugs on the Earth, it pulls the ocean waters toward the Moon, creating a bulge of water.
There's also a bulge on the exact opposite side of the planet as the Earth is pulled toward the Moon and away from the water. That's where high tide comes from.
Low tide occurs in the two places exactly between the two bulges. Since the Earth rotates, we get two high tides and two low tides per day.
The times shift daily due to the Moon also orbiting around the Earth. The sun's gravity also has an effect on the tides, but the Moon's influence is far greater.
For More Information: