It's an area of countryside not built on to allow for rivers to break their banks and flood the wetlands of countryside that nature set aside for it..... It's home for many species of birds and wildlife and not to bad for having a paddle...
If you are a land owner you certainly should understand what it is and if your property lies within. It can affect ability to drill la well, place a septic tank, farm, etc.. While most simply think it mean that if it rains your property may flood it actually has to do with the probability of flood in the next 100 years (starting at any point in time). It's not all about elevation either. My house back home was on top of a small mountain but still in the flood plain. If it rained long enough it was possible that the water would reach my ark. Here in Florida I'm 9 ft. above sea level and can walk to the harbor. Yes, I'm in the flood plain and the evac zone.
Nice Jugs 