I am the new owner of a 1993 Yahoo! 18.5 Sportfisher.  The boat specs say it has a live bait well. I don’t know what to look for. What does it do and what should I be looking for.

Its basically an icebox sized pit in the boat that fills with water. Usually in/under the floor of the boat. Usually has a pump to circulate the water outside the boat through it. Keeps your bait and caught fish fresh and alive.

Thank you!  I found the box at the stern of the boat but there is no pump or hoses to or from the box.  Three holes in the box.
Bottom - drain I think
Middle (largest) - pump suction?
Top - discharge from pump?
Is there supposed to be an inlet for more water?

I should have said SOMETIMES they have a pump. It depends on the boat but what you described where its just a hole or hose that connects to the hull to allow water in is also common. You probably have hookups on the well to allow for the addition of pumps. I don't own a boat. I've just been on several boats that have had them

Be sure to remove all your fish BEFORE you take the boat out of the water. You will regularly hear tales of fishermen who have forgotten this only to find a putrid mess in there days and weeks later!