When you come to the notation divisi in a piece of music, you'll suddenly hear more harmony than before. That's because performers who have all been singing or playing the same part together split into different parts.
Divisi is Italian for "divided." It applies to instruments or voices that play or sing the same notes for most of the piece, but temporarily divide or split up into different parts to enrich a short passage with extra harmony. For example, all the altos normally sing a certain line of music together, but for the divisi, they will split into higher and lower alto parts. The same can happen with instruments — especially if there are many of them, such as violins or flutes.