In many instructional PDFs, page 21 is where students move from warm-ups to expressive playing. Mis Abuelos on that page would likely introduce:

—grandparents who carried the rhythm of old Cuba in their very bones.

The piece is usually structured as a "Cha-cha-cha" or a medium-tempo Latin groove. It allows for expressive, lyrical playing in the melody sections before opening up into explosive improvisational solos. For a student downloading a PDF of this piece, they aren't just downloading notes; they are downloading a narrative of cultural identity and familial love.