Armature wire is used inside of cloth characters to give them a “skeleton” so they can hold a pose. The skeleton is optional.
Thin armature wire is 1/16″ and I use this for the arms of my cloth characters. This brand on Amazon works just as well as the similarly priced version I find in Micheal’s Crafts in the clay section.
Be sure you have a decent pair of wire cutters and needle-nosed pliers to work with any wire armatures.