A World Boss is a massive enemy that scales way beyond the size of an ordinary boss. Unlike ordinary bosses, these titans will often require a team of fully prepared players, and will even exceed a regular party size of four (total in the instance, not your direct party). They also only appear at specific real-life times, helping to ensure there's almost always players available to help out.
Whether you decide to group up or fly solo, once you enter the location where a World Boss spawns, the game will look to place you into an instance with other players on the server.
These instances are timed events, giving the players a short window to defeat the boss for some spectacular rewards, including legendary items! Due to this, it's best to ensure you're at an appropriate level and are decently geared before taking on the challenge.
There's a cap on how many players can join a single fight, so don't be the person weighing your team down if you aren't properly equipped.
Beyond being reasonably geared and leveled, World Bosses will require players to carefully maneuver across the battlefield. They boast a large variety of attacks that come in all sorts of forms. It could be aggressive melee strikes, long ranged attacks, or huge AOE pools.
It's important to be on the move while saving your mobility skills for when you really need them. Having an important defensive ability on cooldown at the wrong moment could quickly spell death.