A self-aligning nut, also known as a spherical nut or leveling nut, is a type of nut

Nut (hardware)

A nut is a type of hardware fastener with a threaded hole. Nuts are almost always used opposite a mating bolt to fasten a stack of parts together. The two partners are kept together by a combination of their threads' friction, a slight stretch of the bolt, and compression of the parts...

 that is used in applications where the fastener is not perpendicular to the surface the nut anchors to. It achieves this action by using a flanged nut

Flange nut

A flange nut has a wide flange at one end that acts as an integrated, non-spinning washer. This serves to distribute the pressure of the nut over the part being secured, reducing the chance of damage to the part and making it less likely to loosen as a result of an uneven fastening surface.MATERIAL...

 inside a specially shaped dished out washer. They are commonly used in the aerospace industry. If this nut were not used the object would have to be spot faced perpendicular to the fastener.