«Respice post te! Hominem te esse memento. Memento mori!» 

This popular Latin has been around for a long time: it has its origins in ancient Rome, where it had a very deep philosipical meaning.

It is believed that in the Roman tradition, after tasting victory in a war, the "Dux" (Latin word for Leader, Commander or General) were paraded through the streets on chariots. As people were idolizing them, they might have even felt like superhumans. To prevent extreme pride and overconfidence, which might lead to a national disaster, the Romans placed a slave in the chariot who would continually whisper "Memento Homo" ("Remember, you are only a man") or "Memento Mori" in the Duces ears, to prevent them from being consumed by "hybris" (excessive pride and self-confidence).

More spefically, they used to say: "Respice post te. Hominem te esse memento. Memento mori!", which means: "Look behind you! Remember that you are but a man. Remember that you will die!" 

It derives that this concept of remembering your mortality isn't to be sad; instead, it intends to clarify and inspire people to do the right things. It is a philosophical tool for attaining clarity, creating the right priorities, staying humble, and unlocking our growth in whatever we do.