"Keep your chin up" = Stay positive and continue. Things go wrong and you can either be sad and get depressed about it or you can "keep your chin up" and keep trying to achieve what you want to. It is normally said to someone who has suffered a disappointment to encourage them to continue :) Se voce queria que eu te explico em portugues - por favor me conta :)

When someone is negative or sad, their head naturally hangs forward and their chin points down. So "keep your chin up" means to stay positive, or at least act positive, don't let your head hang down!