“Overbite” is basically the amount of vertical overlap between the upper front teeth and the lower front teeth. A couple of millimetres of overbite is quite normal and that would be a typical target in orthodontic treatment. Sometimes overbites can be abnormally deep, or even completely absent and the teeth don’t overlap at all (an “open bite”) and there are a few different reasons that these things happen. This can be a combination of problems with the teeth, jawbones and the muscles around them. Your orthodontist can give you an indication of what the problem is, and the reasons behind it as well as the options for dealing with it.
