A screw down crown on a watch is a crown that has threads on the inside and screws down to the case. Screw down crowns prevent accidentally pulling the crown out.
Screw-down crowns are often found on diver’s watches because of how they reduce the risk of the crown being pulled out. Screwing the crown down offers extra protection against bumping or snagging the crown while under water, which would let water enter the case and damage the watch.
Some screw-down crowns have extra gaskets for extra water-resistance.