I’d suggest downloading the app, it helps explain everything.
It means that there are 3 beats per measure and one quarter note equals one beat.
A. The Presentation made recommendations and conclusions.
This is just an educated guess, if the question has more specifications please provide them.
Leonardo Da Vinci is quite the man, he had popular interests not only in art but in science, engineering, mathematics, sculpture, architecture, and anatomy. That’s just to name a few. He popularly made artworks like the Mona Lisa, the last supper, the Vitruvian man, Saint John the Baptist, Salvator mundi and much more. High renaissance / Italian Renaissance era.
I think it became a big thing cuz of the war had a lesson on this in 6th grade