A chink is a very narrow opening or crack. A chink in your bedroom curtains might let enough early morning sunlight in that you'll wake up before your alarm goes off.
Use the word chink to describe the slightest gap between slats in a blind or crevice between two rocks on a cliff. Sometimes the light that shows through this space is also called a chink; "Just a chink of light shone beneath the shade on the airplane window." Chink comes from the Old English cinu, "fissure," and the related cinan, "to crack or split."