intransitive verb. : to take things less seriously.

What does it lighten up mean?

Become or cause to become less serious or gloomy, and more cheerful. For example, Lighten up, Sam—it'll turn out all right. This slangy expression transfers reducing a physical weight to a change of mood or attitude.

Is lighten the mood?

2 : to make less sad or serious A joke lightened the mood.