to make something lighter or brighter:
make lighter[transitive] also brighten something ↔ up
Use blonde highlights to brighten your hair.
The morning sunshine brightened up the room.
to make something more colourful or attractive:
make more attractive[transitive] also brighten something ↔ up
She bought some flowers to brighten the room.
I want to brighten the place up a bit.
to make something more enjoyable, exciting or interesting:
make more enjoyable[transitive] also brighten something ↔ up
His letter brightened my day.
I felt I needed something to brighten up my life.
to shine more strongly, or become brighter in colour:
The stage lights brightened to reveal a street scene.
if the weather brightens, the sun begins to shine and it becomes lighter:
become sunny[intransitive] also brighten up
The sky brightened after lunch.
Let's hope the weather brightens up later.
It brightened up a bit in the afternoon.
to become happier or more excited:
become happy[intransitive] also brighten up
His eyes brightened when we started talking about money.
She brightened up a bit when she saw us.