In the ocean near the moon, the moon's attraction causes the seawater to rise, resulting in a high tide. The ocean on the opposite side of the moon has a weaker pull, so the water is left behind, and this side also has a high tide. This is how seawater gathers on both sides of the earth, resulting in a high tide. The ocean in the middle of the two tides is ebbing because of the decrease in seawater.
Not only the moon, but also the sun pulls seawater by gravitational force. However, depending on the position of the sun and moon, their gravitational pulls overlap and sometimes cancel each other out, causing the tides to rise and fall in different directions.
When the sun, moon, and earth are in alignment, their gravitational pulls overlap, resulting in the largest ebb and flow. This is the case at the new moon and full moon, and is called "high tide.