I am still a newb to the lightweight scene, too. A stove jack is kind of like a thimble for a wood stove in a house. It is where the stove pipe makes the exit out of the tent. This stove jack, and most others, is made of a fiberglass cloth material impregnated with high heat silicone to waterproof it. Here is a link on the installation of the stove jack that I used on this shelter:
http://seekoutside.com/instructions/stove-jack-installation/NT
