A show-stopper is a performance so good that the audience claps and cheers until the show has to pause — the number that stops the show. People also use it more loosely for any moment or thing that is extremely impressive: a song, a dress, a speech, a goal. That is the heart of the word in entertainment, and the related adjective is show-stopping. Though actually, in offices and software, people also call a show-stopper a problem so serious it would halt the whole plan. That is a different stretch of the same picture, so listen for which one the speaker means.