Image: WowheadDuring the WoW: Forever Launch overview, Blizzard revealed that Tier Tokens will unlock a series of quests that ends with the player earning a Tier Piece.
Forever Tier Set Overview.
In the Forever Launch Overview video, the final section discusses raids and what to expect. Raids will once again drop Tier pieces which include bonuses for equipping multiple pieces of the same set. When talking about Tier Tokens, Blizzard revealed some details about how Tier Tokens will work.
Vanquish the raid bosses, and you'll be able to claim rewards that far exceed what you've come across previously. Acquire a Tier Token and you'll open a new adventure for you or any other ally lucky enough to claim their prize. With these rewards, you'll need to seek out how to start a quest that ends with an adventurer earning a Tier Piece, one of the greatest items one can acquire for their given class. Much like dungeons and raids, you won't be able to complete these quests completely on your own. So keep in close touch with your fellow adventurers so that you all can achieve new heights of power.
From this description, getting your Tier Piece looks to be more involved and will require multiple quests, with the first step being getting a Tier Token. Once you have a Tier Token, there appears to be an involved quest line with some steps requiring group content.
Source: Wowhead ↗