A Lock washer exerts a spring force between the fasteners head and the substrate, which maintains the washer hard against the substrate and the bolt thread hard against the nut or substrate thread, creating more friction and resistance to rotation.
These washers create a permanent installation and will lock on plastic, steel etc.

For CALIFORNIA residents only:

WARNING: This product contains chemicals known to the State of California to cause cancer and birth defects or other reproductive harm.
Please note that by posting this Proposition 65 warning, we are notifying you that one or more of the Proposition 65 listed chemicals may be present in a product. With Unique Bargains’ products, the exposure to these chemicals may be of no significant risk to you, but out of caution, we have elected to place this warning on our site to make sure our customers are informed.

You can find more infomation at http://www.oehha.ca.gov/proposition-65/about-proposition-65.