It means to put two light bulbs in each one of your ears and then go walk across Mood City... =D Just joking, don't kill me... ^^ Actually, the real meaning is to take things with a little bit more of sense of humor (for example: my stupid joke over there :) ), to relax if you are being too intense about something. Don't be stressed. Be happy. Lighten up ! ^^

If a conversation or situation is too gloomy or serious we sometimes say this to change direction in the conversation or situation.

To make things more relaxed and happy. "We've been talking about world politics long enough. Let's lighten up the mood. How are your dance lessons coming along?"

"gelirken Malibu aldım, uyku tulumumu da getirdim, gece sizdeyim. bu arada, CouchSurfing üyeliğin var mı?" ben buna "lighten up the mood" derim, ama sen bilirsin.

It's often used in relation to music, where an upbeat or happy song can make things more pleasant.