Anything described as antisubmarine is designed to defend against submarines. Antisubmarine tactics are part of military strategy.
A submarine is a sea vessel that goes under the water, while words that start with the prefix anti are opposed to something. Therefore, antisubmarine plans, gadgets, and personnel are all designed to defend against submarines. An antisubmarine plan tries to figure out and then repel every way a submarine could attack a position. An antisubmarine net is designed to stop a submarine's progress underwater. Naval officers know a lot about antisubmarine strategy.