The sea water rises and falls (recedes) twice in a day. When the sea water advances it is called "Tide' and when it recedes it is called 'Ebb'.
At the time of a high tide, the surface of the sea water rises and rushes towards the shore. During ebb, the surface of the sea water recedes and goes away from the shore. The tides and ebbs occur due to the gravitational force of the sun and the moon. Tides and ebbs do not occur uniformly on all days and at all places. On the Full Moon (Poonam) and on New Moon (Amavas) days the spring (high) tides occur. On the middle days (seventh and eighth days), neap (low) tides occur.