French drain is perforated pipe surrounded by gravel. We also use easy flow which is perforated pipe surrounded by Styrofoam peanuts. Both of these work extremely well in a variety of soils.
The job of the French drain is to collect underground water, sub surface water. So if you have an area of the yard that stays flooded or saturated for long periods of time, a French drain may be needed to help dry out the area.
The same type of system also works on foundation waterproofing. Both in crawl spaces and exterior foundation walls.
Please see selected videos below to help understand how French drain works and how it is installed.