The popping sound that you hear is your body's joints releasing gas that existed prior to the expansion of the joints. Every body has bones. Connecting those bones are ligaments and other soft tissues. As an example, if you've ever seen someone pop their knuckles, they essentially stretch this soft tissue so that a substance inside the joints between the bones releases a gas. The faster the release, the louder the sound.
Is it bad for you?
When done in moderation (like a chiropractor would recommend), getting adjusted is safe and part of a wellness approach to your health.