I can crack my ankle really loudly. why is that and is that bad?

Burst outloud: Typically cracking of the joints is caused by bursting of gas bubbles in the joint fluid. The gas is normally dissolved in the joint fluid. When the joint is stretched, the pressure is lowered in the joint. This lowered pressure causes the gas to come out of solution violently causing a "pop" or "crack". I know of no evidence that this is bad. In fact, it usually makes the joint feel better.

No: The ankle joint has many ligaments surrounding the joint capsule, which also has joint fluid inside the capsule. There are also many tendons coursing down and around the joint (which also have fluid surrounding them). All of these structures can contribute to the "cracking" we hear... which is typically not dangerous.