The idea is that the navy will be hiding pods hidden deep in the ocean so that when the need arises, they will be activated and will be able to unleash a surprise attack on the enemy. Dubbed as Upward Falling Payloads (UFPs), these pods will launch drones that could either attack the enemy or provide surveillance.
Alternatively instead of housing a drone, these pods could also contain weapon systems such as missiles or maybe a naval mine that would surprise enemy ships due to their sudden deployment. That being said, one of the issues right now is figuring out how to maintain these pods in the depths of the ocean for extended periods of time, communicating with these pods, and how they are able to survive due to the extreme pressure in the depths of the ocean.
As it stands, DARPA is still working on the tech but they expect that in Phase 3, they will be able to run a sea demonstration of how these UFPs communicate, rise to the surface, and the different types of payloads they are able to deliver.