It's just called a "shaft log." It has a seal around it composed of a sorta rope like material which is packed under a nut. As you tighten the nut, it compresses the packing material, sealing it off. The only maintenance required is to check the seal periodically, to see if it's leaking at the proper rate.
When sitting still, it usually won't leak at all. When moving, it should drip one drop every 15-30 seconds or so. In fact, it must leak a little. The water lubricates the shaft.
You adjust the tightness of the nut against the packing material to control the drip rate.
There are also dripless seals that some people install or order as options, but the vast majority of inboard type boats have the standard shaft log, which needs the periodic adjustment. it's very easy, and shouldn't take over a few minutes.
My owner's manual says to check it every 50 hours.