When Sara was recording water levels in the ocean once every hour for 24 hours, she will notice that the water levels are slowly rising twice a day, and they are dropping slowly twice a day. Sara is witnessing tides. The tides basically represent the rise and fall of the levels of the water because of the influence of the Moon's gravitational pull.
The Moon is the closest object to Earth, and with its gravitational pull, it manages to influence the water on Earth. The side of Earth that is facing the Moon and the opposite side, experience water rise because the Moon is pulling the water toward it. The sides of Earth that are not exposed to the gravitational pull of the Moon experience drop in the water levels because the water is pulled on the other two sides. When the water rises we have a high tide, and when the water drops we have a low tide.
