sorry for the noob question but where can i find the description of this?,i just cant find it anywhere, i looked on weapon qualities, conditions but nothing ><

All references to it, but one, in the Beta have been changed in the errata as far as I can see. See the errata for the changes in the From Beyond, Overwatch, Suppressive Fire, and Into the Jaws of Hell.

The only reference remaining for pinning is for the Malatant on page 301 of the Enemy Within section.