A jam nut is often used when a nut needs to be locked in place without clamping against another object.
The jam nut essentially acts as the "other object", as the two nuts are tightened against each other.
They can also be used to secure an item on a fastener without applying force to that object.
This is achieved by first tightening one of the nuts onto the item. Then the other nut is screwed down on top of the first nut. The inner nut is then slackened back and tightened against the outer nut.
Hex Jam Nuts 18-8 Stainless Steel - 1/4"-20 (7/16" Flats x 5/32" Thick) Qty-250