Polders are low-lying areas of land that have been reclaimed from water, typically found in countries like the Netherlands. They are created by constructing dikes to hold back water from rivers, lakes, or seas, allowing the land to be drained and used for agriculture or habitation. The process of creating polders involves pumping out water and managing the water levels to keep the land dry. This technique has been essential for managing land in regions where natural water levels are high, helping to expand usable land and protect against flooding.