There are two ways to fight something far bigger than you: a world boss the entire server attacks at once, or a raid your own party chooses to start.
World bosses
A world boss spawns on a schedule and is announced to everybody online. It is one shared enemy with one shared health pool, so a hundred players hitting it are all hitting the same thing. Damage is tracked per player and shown on a live board while the fight runs.
Rewards are proportional to the damage you personally dealt, so joining late is still worth doing and nobody can be locked out by arriving second. The three world bosses currently in the game are The World Devourer, The Gilded Colossus and The Storm Serpent, each with its own signature drop.
Raids
A raid is the small-group version. Rather than waiting for a server-wide spawn, a party commits to a boss together and fights it as a group. It is the same combat system, sized for five people instead of a hundred.
Clan bosses
Clans have their own target, Vargath the Wallbreaker, fought by the clan rather than the server. See clans.
All of it is server-authoritative: the server decides every hit, so a boss fight resolves identically whether you are watching it or not.