Starting from the front, there's an engine, then a gearbox, on the back of the gearbox is the propellor shaft and on the end of this is the propellor.
Clearly the engine and gearbox are in the boat and the prop is in the water. The prop shaft starts in the boat and ends in the water, so it has to go through a hole on the hull, and something has to stop the water comong in through this hole.
Surrounding the propshaft is the propshaft outer - a tube with seals at the end - the shaft runs through it, and the tube is welded to the hole in the hull.
Because the shaft is rotating, the seals can't be 100% watertight so the tube is filled with grease.
That's the background, now the answer to the question.
Gradually grease leaks out of the shaft, so the stern greaser is there to top it up; when you turn it round, it squeezes a bit of grease into the tube.
Sorry if the first part is more simple than you need, but if you don't understand that, then the last bit wouldn't make sense.
