Got it. Thanks!_ Check out this site
http://www.tencate.com/smartsite.dws?lang=en&id=2753
This is one of many that are made. For installations like at Cajun Field, it would be a non-woven polypropylene material used to separate the drainage rock and underlaying soils.
Over time, the movement of water would cause the soil to migrate into the rock. Since the rock is used for drainage, the soil would plug the drain. The geotextile lets water through but not the soil the water is carrying (it is sometimes called "filter cloth" because of this property). Also if the soil is migrating, its leaving its original position resulting in a void, in this case under the artificial turf (not a good thing). So, the water is drained away in the rock and the soil stays in place which is exactly what you want to happen.
I'm sure you have seen the "silt fences" around construction sites. They are there to keep mud from leaving the site. Usually the silt fence is a woven (like burlap) material because it has to withstand wind and the force of any water flow against it. _