"Multiplayer quests" refer to collaborative missions or challenges that can be completed by a group of players together, rather than by an individual player. Full definition
But if you want to actually set up specifically to play a co-op multiplayer quest with friends, there are specific ways to set that up. (twinfinite.net)
But I suppose adding multiplayer quests will be nice for the people playing this game. (squareelite.com)
At a certain point, an option for multiplayer questing unlocks, but individualists will definitely want to cultivate a team that complement's their play style. (tech-gaming.com)