In a crescendo, the music is getting louder. There's often a crescendo in a large group of talking people, too.
This word comes from classical music, where it's very important how loudly the instruments play. If a tuba is crescendoing at the wrong time, then a quiet piano part might not be heard at all. The crescendo is important in all kinds of music, because volume — how loud something is — is one of the main features of music. If you are whispering and gradually raise your voice and then end up shouting, that's a crescendo as well.