The beauty of the quick connect fitting is its simple design. It's comprised of three individual components: a collet, an O-ring, and the body. The body of the fitting conceals the O-ring and collet on either end. Your tubing is meant to be inserted through the collet, through the O-ring, and into the body of the fitting. These three components create the water-tight seal.
As pressure increases inside of the tubing, the water or air forces the collet into a groove on the inside of the fitting. The grooves compress the collet so that the little teeth on the inside of the collet dig into the tubing, making it virtually impossible for the tubing to disconnect. So, the harder you pull on the tubing, the tighter the collet holds onto it. As the collet continues to hold the tubing in place, the O-ring seals the space between your tubing and the fitting that it is connected to, making it water-tight. The most common reason for experiencing a leak with a quick-connect fitting is that the tubing did not get pushed into the fitting all the way.