A passerby is someone who happens to be walking past a place. They are not there on purpose; they just go by. If a window breaks, a passerby might see it. The careful spelling is often passer-by, and the plural is passers-by — the extra s goes on passer, not at the very end. Though actually, you will also see passerby written as one word, as here, especially in American English. It is an ordinary noun, a little more formal than just saying “someone walking past.”