In my opinion, it has different meanings accroding to the different situiations.For instance , if someone just encountered the failure or something else which made him in low spirit,the sentence means "To be cheerful".When someone is just affraid of something ,the sentence means " To be brave!".

It means to gain courage. You would normally say it to someone who appears afraid. This person has no courage. Thus, you tell them to find courage, which English speakers tend to believe lies deep within everyone.