Tides are the way the sea rises a little then falls back every 12 hours or so.
When the tide is flowing it is rising. When the tide is ebbing it is falling.
Tides are caused by the pull of gravity between the Earth, Moon and Sun.
The mutual pull of the Moon’s and the Earth’s gravity stretches the Earth into an egg shape.
The solid Earth is so rigid that it stretches only 20 cm.
Ocean waters can flow freely over the Earth to create two tidal bulges (high tides) of water. One bulge is directly under the Moon, the other is on the far side of the Earth.
At high tide, the sea rises up the shore and dumps seaweed, shells and drift wood. Most coasts have two high tides and two low tides every day.
As the Earth rotates every 24 hours the tidal bulges stay in the same place under the Moon. Each place on the ocean has high tide twice a day. The Moon is moving as well as the Earth, making high tides occur not once every 12 hours but once every 12 hours 25 minutes.
The continents get in the way, making the tidal bulges slosh about in a complex way. As a result the timing and height of tides vary enormously. In the open ocean tides rise only 1 m or so, but in enclosed spaces such as the Bay of Fundy, in Nova Scotia, Canada they rise over 15 m.
The Sun is much farther away than the Moon, but it is so big that its gravity has an effect on the tides.
The Moon and the Sun line up at a Full and a New Moon, creating high spring tides twice a month. When the Moon and Sun pull at right angles at a Half Moon, they cause neap tides which are lower than normal tides.
Neap tides occur when the Sun and Moon are at right angles to each other and pulling in different directions.
Spring tides occur when the Sun and the Moon are lined up and pulling together. High tides happen at the same time each day on opposite sides of the Earth Oceans Tides.