A Med-Bay, assumedly short for Medical Bay, is a bed or gurney-shaped device that humans can use to remedy any ailments or problems related to their health.
The Med-bay is seen frequently throughout the movie. The idea is that a human who is registered to use the Med-Bay simply has to lie down on it, and then the Med-Bay will automatically scan for any diseases, aliments, or conditions, and automatically rebuild the body to remove the problem. The Med-Bays are seen to be able to rebuild broken bones, aging hair or face wrinkles, even cancer and rebuilding a normally fatal injury as long as the brain remains intact.
It is unknown in the movie exactly how the Med-Bay is able to do what it does, whether it uses nuclear energy or extreme chemical application, or whether or not it uses electricity. However, it is assumed the Med-Bay can be used frequently and by many different sorts of people at once, without faulting, so movie fans have often pointed out the Med-bay as an analogy or a reference to privilege or given wealth, showing how the rich have extensive abilities to help the poor, regardless of whether or not they choose to.