Meaning of “lighten” - Learner’s Dictionary

lighten

verb uk us /ˈlaɪtən/
LESS SERIOUS [ I, T ]

If a serious situation lightens, it becomes less serious, and if something or someone lightens it, they make it less serious:

Her mood lightened a bit when I asked about her holiday.
He tried to lighten the atmosphere by telling a joke.
lighten the burden/load

to reduce the amount of work or trouble someone has to deal with

LESS DARK [ I, T ]

to become less dark, or to make something less dark:

The sun had lightened her hair.

(Definition of “lighten” from the Cambridge Learner’s Dictionary © Cambridge University Press)