Often called split lock or split ring washers, spring washers are locking washers that prevent a fastener from loosening as a result of vibration. They have a split in the washer section that when tightened each side of the split bites into the nut and the mounting surface. This  prevents any anti-clockwise rotation from occurring under vibration. Spring washers are usually used on the nut side of the fastener. These are the ‘square section’ type which refers to the end section of the split in the washer, which in this case is square.