The sea levels are continuously rising and lowering. A tide is a cycle of the sea level changing. The maximum water level is called the high tide, and the minimum water level is called the low tide. The tide between high and low tide is called the ebb or falling tide. The time between low and high tide is called the flow, flood or rising tide. The tide, similar to many things on Earth is impacted by the external gravatational forces. If we were to ignore the external fordes, the gravatiational force is directed towards the centre of the Earth, we would have no net lateral forces and therefore there would be no flow of water. The external forces that impacts the Earth, are the gravatational pulls of massive external bodies such as the moon or the sun. The primary changing gravatational field is the Moon. The secondary is the Sun. The sea level rises to the spot where the Moon is closest, and the sea level decreases on spots which are further away from the Moon. However, the Earth itself is also subject to the gravatational pull of the Moon. – (http://www.deltawerken.com )

“…the gravitational pull of the moon and the sun affect the Earth’s tides on a monthly basis. When the sun, moon, and Earth are in alignment (at the time of the new or full moon), the solar tide has an additive effect on the lunar tide, creating extra-high high tides, and very low, low tides — both commonly called spring tides. One week later, when the sun and moon are at right angles to each other, the solar tide partially cancels out the lunar tide and produces moderate tides known as neap tides. During each lunar month, two sets of spring and two sets of neap tides occur (Sumich, J.L., 1996).”

(https://oceanservice.noaa.gov/education/tutorial_tides/media/supp_tide06a.html)