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.