A drip tip is essentially a hollow tube that takes the place of the cart. The hole is quite large, usually funneled, so that by holding the PV straight up and down, one can drip
juice through this mouthpiece directly onto the bridge of the atomizer.
They came about because so many people were fed up with the quick flavor and vapor loss of the old atty/cart combo, they just chucked the filler in the carts, put a few drops right on the bridge, and used the empty cart as a mouthpiece only. Repeat the drops every 7 drags or so, when the flavor drops off & vapor gets weak. The drip-tip helps by not needing to remove it every time you drip.
They're generally really ergonomic... I use them in place of the little rubber stopper on a standard soft tip cartomizer, as they just feel better between my lips, and every hour or so when I want to top off my carto, the drip tip is a no-fuss to remove and replace (compared to that little stopper).