what is Barrier Stretch Fabric?

PI's site has always been a little confusing. Here's a PI video about the barrier fabrics:

http://www.pearlizumi.com/publish/content/pi_2010/eu/en/index/0/TECHNOLOGY.html

Look at the 3rd one down, called "Pearl Izumi Barrier System"

Basically, a "barrier stretch" vest should be a wind and water resistant fabric, like a typical polyester or nylon windshell material (probably with some mechanical stretch to it). If the vest says "Barrier WXB", then it is a 2.5 layer system, more like a GTX-type system. The "thermal" stuff is brushed, like a lightweight fleece and the "softshell" line has a brushed inner with a barrier outer.

Hope that helps and doesn't confuse more!

Or look here: http://www.pearlizumi.com/content/pearlizumi/united-states/en/home/p–i–technologies/innovation-since-1950.html