If you spend an entire day at the beach, it is pretty obvious to notice that the tide changes throughout the day. Sometimes it comes up all the way to your feet while you lay in your beach chair, while other times the tide can be so low it seems like the water is a mile away from you. Why you ask? The answer is somewhat simple: it has to do with the moon and gravity.
As the moon orbits the Earth, the strength of gravity between the two objects increases and decreases. The force of attraction that the moon has on Earth changes depending where you are on Earth. In places where the force of attraction is strongest, the tides will be highest as the gravity from the moon is literally trying to pull Earth apart. Technically this force of attraction effects all of Earth, but you can only really notice it through tides as the force is not strong enough to move land.
Because of the variance of the force of attraction that the moon has on Earth depending on where you are, the tide fluctuates different amounts around the globe. Factors like latitude, the direction the coastline faces, and the shape of the land around the water all impact the variance of the tides.
Check out these cool pictures of high tide and low tide at the abbey of Mont-Saint-Michel, France