A tidal island is a piece of land that is connected to the mainland at low tide at which time they can be reached on foot. At high tide however it is completely cut off from the mainland and becomes a real island.
Tidal islands are sometimes connected to the mainland by a man-made causeway allowing easy access for visitors wishing to cross over but even these can get submerged by the sea. So always check the tide times before you visit, or get ready to swim back.
(thanks Bosko)