Your joints, including those in your knuckles, are surrounded by a membrane called the synovial membrane, which forms a capsule around the ends of your bones. Inside this membrane is synovial fluid, which acts as a lubricant and shock absorber so your bones don’t grind together when you move.
When you “crack” your knuckles, or any other joint, it expands the space between your bones, creating negative pressure that draws synovial fluid into the new gap.
This influx of synovial fluid is what causes the popping sound and feeling when you crack a knuckle. If you continually crack your knuckles, the synovial membrane and the surrounding ligaments will loosen, making it easier and easier for your joints to crack.