Your joints are cushioned by synovial fluid containing dissolved gases, mainly carbon dioxide and nitrogen. Stretching or bending the joint past its normal range briefly drops the pressure inside the joint capsule, and that sudden pressure change causes a gas bubble to rapidly form and collapse (cavitate) — the pop is the sound of that bubble collapsing, similar to how cavitation makes noise around a boat propeller. It takes roughly 20 minutes for the gas to redissolve before you can crack the same joint again.