https://www.corelearn.com/what-we-do/math-discourse-whitepaper/

Learning mathematics is not a spectator sport. The rigorous mathematical knowledge sought for at all levels of instruction require deep thinking and persistent sense making from students. Communication about mathematics among students and between students and the teacher is the vehicle for bringing thinking to the surface, clarifying ideas, moving ideas forward, revealing misconceptions, and making key mathematical connections clear, transferable, and memorable. Mathematical discourse is the verbal and written communication that is centered around deepening thinking about and making sense of mathematics.