Complex sentence: Couperin's collection of piano pieces "pièces de clavecin" is organized into 25 "sequences" ("ordres"), each of which consists of several pieces, with a title for each piece.
Split into simpler sentences: Couperin's collection of piano pieces "pièces de clavecin" is organized into 25 suites for which Couperin used the term "ordre" in the sense of "sequence". Each sequence consists of several pieces, with a title for each piece.