What Causes Tides?
The Short Answer:
High and low tides are caused by the Moon. The Moon's gravitational pull generates something called the tidal force. The tidal force causes Earth—and its water—to bulge out on the side closest to the Moon and the side farthest from the Moon. These bulges of water are high tides.
High tide (left) and low tide (right) in the Bay of Fundy in Canada. Image credit: Wikimedia Commons, Tttrung. Photo by Samuel Wantman.
High tides and low tides are caused by the Moon. The Moon's gravitational pull generates something called the tidal force. The tidal force causes Earth—and its water—to bulge out on the side closest to the Moon and the side farthest from the Moon. These bulges of water are high tides.
As the Earth rotates, your region of Earth passes through both of these bulges each day. When you're in one of the bulges, you experience a high tide. When you're not in one of the bulges, you experience a low tide. This cycle of two high tides and two low tides occurs most days on most of the coastlines of the world.

This animation shows the tidal force in a view of Earth from the North Pole. As regions of Earth pass through the bulges, they can experiences a high tide.
One thing to note, however, is that this is just an explanation of the tidal force—not the actual tides. In real life, the Earth isn't a global ocean, covered in an even layer of water. There are seven continents, and that land gets in the way. The continents prevent the water from perfectly following the Moon's pull. That's why in some places, the difference between high and low tide isn't very big, and in other places, the difference is drastic.
That explains the first high tide each day, but what about the second high tide?
The ocean also bulges out on the side of Earth opposite the Moon.
The tidal force causes water to bulge toward the Moon and on the side opposite the Moon. These bulges represent high tides.
Tidal force = Moon's gravitational pull in a specific location on Earth — Moon's average gravitational pull over the whole Earth
The result of the tidal force is a stretching and squashing of Earth. This is what causes the two tidal bulges.
Arrows represent the tidal force. It's what's left over after removing the Moon's average gravitational pull on the whole planet from the Moon's specific gravitational pull at each location on Earth.
scijinks.gov
The Short Answer:
High and low tides are caused by the Moon. The Moon's gravitational pull generates something called the tidal force. The tidal force causes Earth—and its water—to bulge out on the side closest to the Moon and the side farthest from the Moon. These bulges of water are high tides.

High tide (left) and low tide (right) in the Bay of Fundy in Canada. Image credit: Wikimedia Commons, Tttrung. Photo by Samuel Wantman.
High tides and low tides are caused by the Moon. The Moon's gravitational pull generates something called the tidal force. The tidal force causes Earth—and its water—to bulge out on the side closest to the Moon and the side farthest from the Moon. These bulges of water are high tides.
As the Earth rotates, your region of Earth passes through both of these bulges each day. When you're in one of the bulges, you experience a high tide. When you're not in one of the bulges, you experience a low tide. This cycle of two high tides and two low tides occurs most days on most of the coastlines of the world.

This animation shows the tidal force in a view of Earth from the North Pole. As regions of Earth pass through the bulges, they can experiences a high tide.
One thing to note, however, is that this is just an explanation of the tidal force—not the actual tides. In real life, the Earth isn't a global ocean, covered in an even layer of water. There are seven continents, and that land gets in the way. The continents prevent the water from perfectly following the Moon's pull. That's why in some places, the difference between high and low tide isn't very big, and in other places, the difference is drastic.
That explains the first high tide each day, but what about the second high tide?
The ocean also bulges out on the side of Earth opposite the Moon.

The tidal force causes water to bulge toward the Moon and on the side opposite the Moon. These bulges represent high tides.
To get the tidal force—the force that causes the tides—we subtract this average gravitational pull on Earth from the gravitational pull at each location on Earth.Tidal force = Moon's gravitational pull in a specific location on Earth — Moon's average gravitational pull over the whole Earth
The result of the tidal force is a stretching and squashing of Earth. This is what causes the two tidal bulges.

Arrows represent the tidal force. It's what's left over after removing the Moon's average gravitational pull on the whole planet from the Moon's specific gravitational pull at each location on Earth.
These two bulges explain why in one day there are two high tides and two low tides, as the Earth's surface rotates through each of the bulges once a day.What Causes Tides? | NOAA SciJinks – All About Weather
Tides are a complicated dance between gravity and inertia.