Stuffing box
Rick - That's what it's called. Looking at your picture, see where the drip is? That thin nut is the lock nut, the big one in front of it is the packing nut. First, soak the whole thing good with WD-40 and let it penetrate. Then use two 10 or 12" pipe wrenches, one on the lock nut, and one on the packing nut, and break them free. You want to turn the lock nut toward the back of the boat, and the packing nut toward the front of the boat.Looks like there's plenty of thread behind the lock nut - after you break them free, you may be able to tighten the packing nut to stop the drip. Then tighten the lock nut on the packing nut. You may need to change the packing material - easy job. Check the archives for a complete description. While you're at it, take a wire brush to that shaft coupling, get the rust off, and spray it with black rustoleum paint.