The player unlocks Mastery mode for Beginner tracks by earning at least one medal on 39 tracks and all medals (except the Ten medal) on 17 Beginner tracks. Other Track Types require unlocking the previous Mastery level and having all medals on more tracks. Each Mastery level unlocked also unlocks an extra tier of upgrades in the Monkey Lab.
Internally, Mastery mode simply adds 1 to each Bloon's rank. As a result, round 85 is completely unchanged.
Additionally, when the Golden Bloon was released, it was considered 1 rank above the Ceramic Bloon, causing a bug where Ceramics were upgraded to Golden Bloons instead of MOABs.
↑"Do you have what it takes to become the ultimate Bloons master? Prove it with the brand new Masteries mode!" — In-game description of the 3.0 update.