"To master the material" means to have a deep understanding and knowledge of a specific subject or topic. It implies being able to fully comprehend and confidently apply the information and skills related to that particular area. Full definition
For pre-K student's, moving to a new school can make it very challenging to learn and master materials in the classroom when you are not in the classroom while moving. (e3alliance.org)
Few teachers can master all the challenges of teaching state standards, designing engaging projects, assessing all students along the way, and intervening effectively when students don't master the material. (edutopia.org)
Just as educators are pushing students to build a huge reservoir of knowledge, they are also focused on having students master material, sometimes at the expense of relevance. (gse.harvard.edu)