A Cutless bearing is a bronze tube, pressed into the strut which hangs down between your rudder and the point at which the shaft goes into the hull. Inside the bearing is a neoprene sleeve which is somehow attached to the bronze, perhaps by some sort of an adhesive.
The strut supports the shaft and prevents it from flailing around as the prop pushes the boat through the water. The Cutless bearing provides a surface for the shaft to rest on inside the strut.
Over time the neoprene wears down and sometimes it breaks loose from the bronze tube. This creates wobble and vibration.
One way to replace the bearing is by removing the shaft, assembling a hacksaw inside the bearing, taking a slice or two out of the bronze, and removing the old bearing. Then, using a big clamp, you press in a new bearing.
If you search this forum for "Cutless" or "cutlass" (which is not really the right way to spell it) you will see other ways of doing it, including one which requires a simple tool but doesn't require removing the shaft--just the prop.
A very good project if you are already removing the engine.