A Hex Nut is a fastener with a threaded hole

A Hex Nut is a fastener with a threaded hole. These are often used with mating bolts to hold parts together. These are commonly found on bicycles, automobiles, and more. But before you jump on the bandwagon and buy one, you should know more about what they are.
Hex nuts are a popular type of fastener and are often used to connect bolts and other components. Because of their hexagonal shape, they are easier to use and tighten than other nuts. They also work through compression and friction with the thread to help fasten them securely. For optimum performance, hex nuts should be long enough to go through two full threads.
Most hardware stores carry a variety of hex nuts for many applications. However, if you're doing a specific project, you may need to order a nut in a different size. You can find these specialty nuts at hardware stores or online. They're useful for projects that require multiple sizes, such as for fastening a motor.
A hex nut is usually made of steel and is available in different strength "grades." Grades are standardized by the Society of Automotive Engineers and range from grade 2 to grade 8, with the lower-grade being Grade 2 and the highest-grade rated at Grade 8. The grade of a hex nut is indicated on its surface.
jinduhardware.com