Light Up Your Life is for lighting 20 candles and lanterns throughout the game. There are 26 scattered across the levels so this one isn't too bad and you can even afford to miss a few. I go over the locations of ALL of the collectibles in Little Nightmares in my video linked below, as well as in my written guide over on my website linked below that.
www.pandallax.com/guides/little-nightmares-100-achievement-guide