A concert is a performance by musicians or singers for an audience. If you're in the glee club, you know about singing in concerts, and maybe you've seen your favorite bands in concert, too.
A concert usually doesn't involve theatrical staging. You can have a concert version of an opera, but that means the singers stand there and sing without sets or props, and without much acting or moving around. Less commonly, concert is a verb (accent on the second syllable), meaning to agree with others on something. That meaning shows up in the phrase in concert — your objections will carry more weight if you act in concert with your neighbors.