Phoenix Feathers are a currency primarily obtained in the endgame PvE content and are used to buy items normally dropped in endgame PvE content as well as Phoenix Ash and Phoenix Tears which are used in important alchemy crafting recipes among other things. Phoenix Feathers are also the alternative currency for Reskins.
Ways to obtain feathers:
1) Endgame PvE Instances (Darkspire, Halls of Helgarth, Celestius)
2) Up to 2000 daily via Keep Raids that have defending and attacking players
3) Epic Dungeons (Caer Sidi, Tuscaran Glacier, Galladoria)
4) Realm Dragons
5) Darkness Falls
6) SI Loops: The high level SI Outdoor zones have named mobs in each realm)
7) Classic Epic Zones (named mobs in the dragon zones as well as Llyn Barfog, Raumarik and Cursed Forest)
8) Passage of Conflict
9) SI Dungeons
Ways to obtain feathers:
1) Endgame PvE Instances (Darkspire, Halls of Helgarth, Celestius)
2) Up to 2000 daily via Keep Raids that have defending and attacking players
3) Epic Dungeons (Caer Sidi, Tuscaran Glacier, Galladoria)
4) Realm Dragons
5) Darkness Falls
6) SI Loops: The high level SI Outdoor zones have named mobs in each realm)
7) Classic Epic Zones (named mobs in the dragon zones as well as Llyn Barfog, Raumarik and Cursed Forest)
8) Passage of Conflict
9) SI Dungeons