The musical instruction tutti means "all together." It's used when all the performers in an orchestra, choir, or ensemble should play the marked passage simultaneously.
Tutti is Italian for "all." It usually comes up in a piece of music after a passage where just one or a few of the voices or instruments have been playing, while the others were silent. Then, where it says tutti, the whole choir or orchestra begins singing or playing, so all the different parts are heard in full harmony. This usually means the music is louder, too. It can create a very effective contrast!