Brightness or the level of light/luminance; can also refer to cheerfulness in a person's character.
部屋の明るさを調整するために照明を買い替えた。
I replaced the lighting to adjust the brightness of the room.
画面の明るさが強すぎて目が疲れる。
The screen's brightness is too high and my eyes get tired.
彼女の明るさに救われた気がする。
I feel saved by her cheerfulness.