Twice a day, the sea moves up and down the seashore. It is high tide when the beach is covered with sea. After high tide, the sea turns and goes back out again. When the sea is far out and the beach is uncovered, it is low tide.
At high tide the seashore is covered.
At high tide, the seashore is covered with water. The high tide line is the highest place the water reaches up the seashore. The tides are caused by the gravity of the Moon and Sun pulling the seawater towards them.
The seashore is uncovered at low tide.
At low tide, the seashore is uncovered and the sea is far out. On some seashore the difference between high and low tides is big. On others it is small. Low tide happens twice in about 25 hours.
Waves are made by the wind.
The waves that move across the sea and break on seashore are made by the wind. When the wind blows over the sea, it pushes and drags against the surface and forms waves. Strong storm winds make huge waves. On days when there is no wind blowing, the sea is calm and the waves are very small.