Okay, so imagine you have a friend who really likes chocolate cake. They like it so much that they want to eat it all the time, even for breakfast and lunch and dinner. But the problem is, if they eat chocolate cake all the time, they might get sick or not be able to do other important things, like go to school or work.
Now replace chocolate cake with alcohol. Some people really like alcohol and want to drink it all the time, but that can make them sick or not be able to do important things. A high-functioning alcoholic is someone who drinks a lot but still manages to do important things like go to work or take care of their family.
But just like eating too much chocolate cake can be bad for you, drinking too much alcohol can also be bad for you. It can hurt your body and your relationships with other people. So it's important for anyone who drinks to do it in moderation and not put themselves or others in danger.