This idiom is used to encourage somebody to not lose courage, to keep trying, and to stay positive when experiencing difficulty. It references a person's tendency to drop their chin towards their chest when upset or down about something. This expression, which can also be shortened to 'chin up,' is so commonly used that it has become something of a cliche. While it is spoken in the form of a command, it is never used negatively as it is meant as a friendly piece of advice or encouragement. As the video shows, it is common in both a social and professional context. Phrases that share a similar meaning include 'hold your head up high,' and 'keep your head up.'