v.1. To make something appear lighter in shade: The artist lightened up the paint by mixing in some white. Those mirrors really lighten the room up.
2. To become lighter in shade: These pants are too dark now, but they will lighten up after they've been washed a lot.
3. To make something more pleasant and less serious or depressing: The jokes lightened up the tone of the meeting. I didn't mean to be disrespectful—I was just trying to lighten things up.
4. To take matters less seriously: Everything will work out fine, so stop worrying and lighten up.