fun fact: popping your knuckles is actually fine to do if you know what you're doing
the pop comes from the fluid between your bones in your joints that acts as padding - as you move, bubbles of air get worked into this fluid, and this makes it less effective as padding and thus your bones can end up grinding against each other. a joint popping (unless it's moving out of place, which is always bad) is caused by these air bubbles popping, allowing that fluid to act as it normally does
the risk comes in if you try to deliberately pop a joint and flex it too far, which can cause damage to your ligaments and is just Generally Not Good. so don't try and force a joint to pop, just flex it around within its normal bounds and see if it does so. if pushing it any further hurts, don't do that