The purpose of a retention pond is to treat and store storm water runoff.
A retention pond is a permanent pool or pond of standing water and eventually the water will empty into a receiving water body.
The retention pond helps to prevent flooding as well due to storm water that could overflow other things and then flood homes and businesses.
It also helps keep bad things out of the water that can get into the water during the storm.
So then the water in the retention pond is stored, treated and then sent back into a lake or other place that is close to the retention pond.