A crack is a very narrow space, chink, or opening. If your favorite mug has a crack in it, it might leak.
A crack can be as thin as a pencil line, or a wider gap, like a crack where your foot gets stuck as you climb down the side of a mountain. Crack is also a verb, meaning "to split or break," and also "to give way under pressure," as when a suspect cracks after hours of being interrogated. An informal kind of crack is a joke: "That crack he made about my haircut really hurt my feelings."