A dike is a structure made of earth or stone that's used to hold back water. If you visit the Netherlands, you can see their elaborate system of dikes, without which much of the country would be underwater.
The verb dike means to enclose with a structure, like a wall or embankment, intended to hold back water. The Netherlands diked an inland sea, used pumping and other water control measures, and was able to reclaim flooded areas. The Dutch have used dikes to prevent flooding since the 12th century, and the word itself has the same roots as the Dutch word dijk.