A crack is any chip that has extended in any direction more than an inch or so. Since chips (pockets of trapped air) are constantly under pressure, anything that adds pressure to the windshield (temperature changes, car wash, slamming the door too hard) can cause a crack-out and split your windshield in multiple directions. This interferes with the integrity of your windshield and reduces the safety of the glass; that is why it is so vital to fix your chips before they spread.