What does the "ritornello principle" describe in Baroque concertos?

The ritornello principle describes the structure of Baroque concertos where the ritornello theme, which is a recurring passage, is contrasted with solo sections performed by the soloist. This principle highlights the alternating nature of the music, where the full orchestral ritornello is interspersed with virtuosic passages played by the soloist. The repetition of the ritornello throughout the piece provides a cohesive framework, allowing the soloist to showcase their skill while still tying back to the main thematic material. This alternating pattern creates a dynamic interplay between the orchestra and the soloist, characteristic of the Baroque concerto form, and is a key feature that aids in the overall structure and progression of the piece.