Tide is the change in the heights of the level of the ocean. It is caused by the pull of gravity of the moon and the sun. Tides are more affected by the moon because the moon is closer to the earth. When the sun and the moon line up, then the sun has more effect on the tide.
Spring tide occurs when the sun and the moon are lined up. This happens twice a month at the "new moon" and "full moon" phases. This causes higher tides. When the sun and moon are not lined up it is called a "neap" tide. The level of the ocean is not as high.
One place where the tides change the most is in the Bay of Fundi, in Nova Scotia. The tide changes about 45 feet from low tide to high tide. The place where the tide changes the least is in the Mediterranean Sea.
Tides are important if you have to go into the ocean. We usually check the tides to see if it is low or high tide and if it's good for fishing or swimming. People in history have had trouble with the tides. Caesar's war galleys couldn't enter the British shore because they did not remember about the tides changing. Famous King John of England lost all of his treasures and some of his army in 1216 and died shortly after that.
Some legends also explain about the changing tides. Here's a good legend about tides: When an angel puts her finger in the ocean, then we have high tide and when she takes it out, we have low tide. There's another legend that describes the tide like the breathing cycle of a whale.