A roll of gray adhesive tape used before the war to seal ducts, leaks and fix just about everything. Post-War they can be used to make weapons.
Duct tape (along with Wonderglue) is the main source of adhesive in Fallout 4. Adhesive is a rather rare and much sought-after element since it's used in almost every crafting recipe. The most widespread form of duct tape is the single roll, but sometimes one can find military-grade or packed variants, which provide even more adhesive.