Trials are encounters where adventurers fight together against extremely powerful creatures. Each trial consist of a single boss fight (a few are broken up into two parts). When the boss is defeated, players are rewarded with story progression, tokens or rare items of sorts. Some trials require a light party of 4 players while others need a full party of 8. Most trials after Level 50 are unlocked first as Hard Modes by progressing through the main story quests; with their upgraded Extreme Mode available through side quests.
A Realm Reborn (Level 1 - 50)
Heavensward (Level 51 - 60)