Cooked all the way through, with no pink or red left inside. We use it almost only about meat, and most often about steak or beef, because that is the meat people can choose the cooking level of. In a restaurant the waiter may ask how you want your steak, and the usual answers go rare, medium rare, medium, medium well, well-done — well-done is the far end, cooked the longest. Note that it is a description of a level, not a judgement: a well-done steak is not a better steak. Many cooks quietly think it is the worst way to eat good beef, because long cooking makes it drier and firmer, so a few chefs will even refuse to do it. Your students should know the trap here: there is another 'well done' — two separate words, no hyphen — that means 'good job!' and is said to a person, not about food. Same sounds, different thing. Written with the hyphen and sitting in front of a noun, it is about cooking: 'a well-done steak'. After the verb it usually loses the hyphen: 'I like my steak well done.' Both spellings are common and nobody will mind. One more small thing: with chicken and pork, nobody says well-done, because those are always cooked through anyway — you would just say 'cooked'.