Ritornello translates to "return" in Italian. A ritornello is a recurrent section of music that alternates with contrasting material. Ritornellos may be the same each return, or they may be varied.
In a concerto grosso, the orchestra will play the ritornello (usually marked "tutti") while the solo part will have the contrasting material (marked "concertino"). In operas and baroque cantatas, the accompanying instruments perform the ritornello while the vocalist performs the contrasting sections in an aria with technically demanding and interpretive material.
Antonio Vivaldi was the first composer to use ritornello. Let’s break down the "Winter" concerto of The Four Seasons to see how ritornello is used.
On the next page is an excerpt from the beginning of the first movement of "Winter". Notice how it features a virtuosic violin solo in the top line.