Shit is one of the most commonly used slang words that mean "poop" or "crap." Or it's used as an emphatic exclamation roughly meaning "oh no!" Despite its commonness, shit is still considered inappropriate.
When you stub your toe or cut yourself, it's pretty hard not to say "Shit!" Still, you'd better try to avoid it in most settings, because shit is an obscenity that many people do not want to hear. It's probably a bad word because poop is generally considered pretty disgusting. So even though we all shit, and we all probably say shit, we need to be careful who we're saying it around.