If you keep your chin up, you are not bowing your head in defeat or sadness. This is a way of telling someone to have a positive attitude.
If someone is going through a time of sadness or trouble, you can say "Keep your chin up - things will get better!"
If someone always has a good attitude about things, you can say "He really knows how to keep his chin up."
keep your faith up you know
maintain a positive outlook.