a floodplain is an area next to a river, stream, or creek that may be covered with water following heavy rainstorms. This plain holds the excess water allowing it to be slowly released into the river system and seep into groundwater aquifers. Floodplains also give time for sediment to settle out of floodwaters, thereby keeping it out of water bodies. Floodplains often support important wildlife habitat and are frequently used by humans as recreation areas.
The 100-year floodplain is the land that will be covered with water during a 100-year storm, and is the accepted limit for protection. Replacing highly developed land with a landuse that is functionally connected to the river, a "hydrologically-active area", would be an improvement. The 100-year storm is an amount of rain so heavy that the chances of experiencing such a storm are one in 100. (Note that those are the chances every year. It is possible to have this unusual storm two years in a row or even more frequently.)