If you go into hospital to have your ingrowing toe nail seen to and when you come out you discover you have been given a heart transplant and your toe nail is just the same as it was when you went in, you have been the victim of medical negligence.
It does not have to be as obvious as that. Lots of people go into hospital and come out feeling not quite as good as they thought they would feel and in some cases even worse than when they went in. Soothing noises are often made in this situation by members of the medical profession.
The patients concerned are not always entirely convinced that this advice is entirely disinterested and they would like to have the matter looked at by somebody more expert and more independent, and if it turns out something is not as it should be they might wish to make a complaint or a claim for compensation for some sort. At the very least they would like to get the problem sorted out.
This is all what lawyers call medical negligence, or sometimes clinical negligence, and a lawyer will be able to help you find your way through this process.