To lighten something is to make it brighter or shine a light on it. You can lighten your dark basement room with plenty of lamps and a coat of white paint.

Besides "make lighter or illuminate," another meaning of lighten is "make lighter in weight." So you can lighten your load by taking all the heavy books out of your backpack. Figuratively, "to lighten" is to be more cheerful or relaxed, as in: "She decided to lighten the somber mood in the room by telling a few of her funniest knock-knock jokes."

Definitions of lighten
  1. verb
    make lighter or brighter
    synonyms: brighten, lighten up
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    make dark or darker
    type of:
    alter, change, modify
    cause to change; make different; cause a transformation
  2. verb
    reduce the weight on; make lighter
    “she lightened the load on the tired donkey”
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    type of:
    disburden, unburden
    take the burden off; remove the burden from
  3. verb
    become lighter
    “The room lightened up”
    synonyms: lighten up
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    become dark or darker
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    cast rays of light upon
    illume, illuminate, illumine, light, light up
    make lighter or brighter
    illuminate with floodlights
    illuminate with a spotlight, as in the theater
    type of:
    undergo a change; become different in essence; losing one's or its original nature
  4. verb
    make more cheerful
    “the conversation lightened me up a bit”
    synonyms: buoy up, lighten up
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    weigh down, weigh on
    be oppressive or disheartening to
    type of:
    show approval or good wishes by shouting
  5. verb
    become more cheerful
    synonyms: buoy up, lighten up
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    type of:
    cheer, cheer up, chirk up
    become cheerful
  6. verb
    alleviate or remove (pressure or stress) or make less oppressive
    lighten the burden of caring for her elderly parents”
    synonyms: relieve
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    type of:
    make less severe or harsh
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