The Master Tier subscription allows a subscriber to turn on content sharing for up to 3 campaigns in which she participates. Each campaign can have up to 12 participants (13 counting the DM).
Turning content sharing on for a campaign means that any content unlocked by any participant (player or DM) within that campaign is available for all players and the DM for use.
We originally announced that a DM has to unlock the content in order to share, but that is no longer the case - anyone in that campaign can unlock content and share it with the other people in that campaign as long as one person (doesn't have to be the DM) has a Master Tier subscription and has used it to toggle content sharing on for that campaign.
To use an analogy:
Dany wants to DM a campaign with her new friends Jon and Davos, and some of her older friends Tyrion and Varys. Jon agrees to host the game weekly at his home up north. Dany owns the Dungeon Master's Guide and the Monster Manual, but she never bought the Player's Handbook because every time she's ever played someone else had it. Tyrion owns the Player's Handbook and the Sword Coast Adventurer's Guide and will bring those books with him to Winterfell each session. Jon picked up Volo's Guide to Monsters somewhere along the way because he's into that kind of thing. Varys hasn't bought any of the books, because he's got a talent for leveraging other peoples' resources. Davos barely wants to play and only agreed to come because Jon asked him to.
When these players meet at Winterfell, they will all get to create characters using options in the Player's Handbook, Dungeon Master's Guide, Monster Manual, Swords Coast Adventurer's Guide, and Volo's Guide to Monsters because all of those books have been brought to the table. Jon of course picks a champion fighter. Tyrion and Varys both choose to be mastermind rogues from SCAG. Davos figures out how great D&D is and really gets into it - he ends up choosing a triton from Volo's because he likes the sea.
In the analogy, Jon's Winterfell is the Master Tier subscription - it brings everyone together in a campaign and allows them all to share their books. Dany and Tyrion can use all the books even though they have only bought a couple themselves. They can share those books with the entire group, even Davos and Varys, who don't own any of them. After Davos gets addicted to the game and picks up Xanathar's Guide to Everything, the whole group could then use that too.
The major difference between the Winterfell campaign and a Master Tier content sharing campaign on DDB is that the books normally would go home with either Dany (as DM) or their respective owners between sessions, whereas every player can access all the books at any time with the Master Tier.