Well, maybe I used the wrong term. I guess more specifically, we use woven geotextile stabilization fabric. It's used for under driveways or large patios to both stabilize the soil underneath but also provide a layer of separation so that the soil (clay, in our case) doesn't come up through and contaminate the gravel base of your patio or driveway.
It's my understanding that if you just lay down 4" or more of gravel and compact it with a plate compactor that later on eventually the soil underneath begins to work it's way UP into your gravel. And over time that destabilizes the gravel, resulting in sinking. So this fabric helps prevent that.
Apparently, it's ICPI standards to use this every time you install a paver patio or driveway. So we've been doing it ever since I found that out. First install, though, I found that the base was clay and because it was winter and it had been raining here, the clay was not hard. It was very mushy. So we laid down the fabric and several inches of gravel and compacted, as instructed. But even then, the gravel was still a little mushy in some areas. So we had to add more gravel, compact more, etc. to finally get it to be stable. That sucked. So I went to my supplier and said, "Hey, this fabric helps, but it was still mushy UNDERNEATH the fabric. So we still had trouble. What do you do when that happens?" He advised me that what they do is lay down a light 1-2" course of gravel and compact it INTO the clay soil. And you keep doing that until the soil gets firm. THEN you add this fabric and THEN you add your gravel base and compact. So that's how we do it now.
It looks and feels just like pond underlayment fabric. About 3/8" thick woven fabric.
Here is a site about it. This is the stuff we use;
http://www.usfabricsinc.com/products/drivewayfabric.shtml
.