In naval parlance, vertical replenishment or VERTREP is the practice of delivering stores and supplies to ships at sea via helicopter. A cargo helicopter will pick up a parcel of cargo pallets using its ventral hook, and carry them over to the destination ship (generally to its flight deck, though occasionally to a spot on the forecastle). Fragile cargo or personnel will be carried inside the helo instead of underslung.
Most US Navy supply ships have a hangar for two CH-46 Sea Knight, MH-60 Seahawk or Super Puma helos, as do many supply ships in NATO navies. Other helicopters not specifically designed for VERTREP, like the MH-60R Seahawk ASW helo or the Westland Lynx, can be used for cargo transfer as well. This is common when a supply ship that lacks an onboard aviation element conducts replenishment with a warship (like a cruiser or amphibious assault ship) which does have one.
While VERTREP is usually performed by supply ships, other aircraft-carrying ships can also fill the role. Sometimes, an aircraft carrier will receive a load of cargo, then disseminate it to its escorts, freeing up the supply ship to replenish another battle group. This tactic is common when there are drastically more warships in a given area than there are logistics vessels, an all-too-common situation.
The actual flying involved in a VERTREP isn't easy. It requires a pilot to maintain a low hover in order to retrieve the cargo parcels, and then fly to the target ship and drop the cargo without damaging it. This requires careful attention, since acceleration will tend to cause the suspended cargo to swing. For these reasons, vertical replenishment is generally only performed in calm seas with low relative winds. Despite this, it can be performed in a broader range of conditions than connected replenishment, and is dramatically faster than CONREP when only a small amount of cargo must be transferred. Also, with a sufficiently efficient flight deck crew, it's possible to VERTREP multiple ships concurrently. I've personally been involved (as deck crew) on a VERTREP including two supply ships, a carrier, a destroyer, a cruiser and five helos. (Two from one supply ship, two from the carrier and one from the cruiser).