Shit is a slang word for feces. Shit is a swear word (an offensive, rude, or impolite word) to most English speakers. It can mean bad things other than feces, depending on the context. It can be used as an angry exclamation, which is something said loudly or with strong emotion. It can be an adjective that means low quality or worthless. It can be a verb that means to produce feces (poop). The related form bullshit is a slang word with meanings that include nonsense and lies.
The general etymology of this is from the three words (of different languages): Old English "scitte", Dutch "schijiten", German "schiessen".