The term tide refers to periodic change of the movement of seawater (to and from the mainland). This water movement is due to the position of the moon relatively to the earth. When the water moves towards the shore then there is high tide and while when the water moves away from the shore then there is low tide. We have 2 high tides and 2 low tides each day (24 hours),  but the exact time of the phenomenon varies from day to day.
In Greece, the phenomenon of the tide can be observed in Chalkis, at strait of Euripus, and in Loutraki at Vouliagmeni Lake.