Spring Lock Washers are a common form of Washer.
They help with the prevention of fasteners becoming loose during vibration. Spring Lock Washers contain a split so that when tightened each side, they bite into the nut and mounting surface. This helps against any anti-vibration rotation that may occur during vibration.
![Spring Washers](https://springfasteners.co.uk/wp-content/uploads/2017/01/Spring-Lock-Washers-1024x498.png)