According to the wiki:
The Mastery system is an account-wide progression system available once players reach level 80, the maximum level available.
Mastery training progress is also account-wide, allowing progression to be made on any level 80 character that gains experience.
When you unlock the mastery system (being level 80, owning Heart of Thorns, and completing the story chapter 'Torn from the Sky'),the Mastery panel will appear as the last (lowest) tab on your Hero panel, and it can also be accessed by clicking anywhere on the XP bar. XP will be applied to the active track.
As David Yell suggested, there are two Mastery sets, one for both Tyria (Core Game) & the other for Heart of Thorns (Expansion); these each contain various tracks.
Also, as MrLemon has rightly stated, the prerequisite for the Spirit Vale track (1st Raid Wing) is to ensure that the Vale Guardian has been killed.