Pertex is a performance fabric made of Nylon, like many other outdoor/tech fabrics, which originates from England and was made available in 1970. It was a product of a collaboration between mountaineer, Hamish Hamilton and the textile mill Perseverance Mills Ltd. in Manchester. Nowadays the Japanese company Mitsui & Co produces the fabric, after they gained the rights to the fabric, after Perseverance liquidated in 2005.
There are different classes or grades of Pertex fabric, but the Classic is very similar to the one, that Hamish Hamilton helped develop, which is known for its light weight (even though it's the heaviest of all the grades of Pertex fabric), breathable qualities and wind resistance. It all comes down to the capillary action, that goes on within the fabric. This moves moisture along the fabric fibres and spreads it over the surface, and then it evaporates faster.
This effect is reached by combining two different yarns each with different properties. Basically the yarn on the inside has larger filaments than the outside yarns which have smaller filaments. The capillary action is driven by the temperature difference between your body heat and the outside air. It moves the moisture from the inside to the outside because the finer yarns on the outside have a greater surface, that will absorb more moisture.
The tight weave of the finer yarns is the main reason to the wind resistance of the fabric. It isn't weather resistant however, but it will take some rain. If you're looking for the lightest Pertex options, you should go for the "Quantum" or "Microlight" probably, but these fabrics don't quite have the abrasion resistance of the Pertex Classic.
Lately Pertex has been used by more fashion oriented brands like A Bathing Ape and White Mountaineering, both out of Japan, so it isn't solely for outdoor brands like Rab.