Climbing nuts are thin wedges of metal that are slotted into constrictions in cracks to provide protection when climbing traditionally outdoors. They can also be called wires, rocks or stoppers, and are the staple of any trad or aid rack as they provide solid protection when placed correctly.
Nuts are classed by climbers as passive protection as they have no moving parts. They can help you place protection where a cam (active protection) might not work, and they are a lot more cost effective too! (Read more about cams and hexes here.)