The gravity of Earth’s moon, and to some degree, the sun, pull on the Earth, creating a bulge—most strongly in the water. As the Earth spins through the day, we rotate through the bulge—thick, thin, thick, thin: the thick parts of the bulge are the high tides, the thin parts of the bulge are the low tides.
Tides move up rivers, too, although they’re delayed as they travel east. And because the continents “get in the way,” the tides aren’t the same everywhere.
Click here for more about tides in general.