Loom
The Loom is used to turn threads into cloth. The command to collect spider webs is also set here.
The Loom's functions are automated by default so any weavers will automatically collect spider webs and weave thread into cloth (if you have any). You can change this behavior in the "set workshop orders" section of the standing orders screen.
One loom only generates one Collect silk job, multiple looms let you harvest silk faster if you have enough dwarves with the weaving labor enabled and enough accessible webs.
Labors: Weaving
In real life
Source: https://commons.wikimedia.org/wiki/File:The_Korkosz_Croft_in_Czarna_Góra_01.jpg