On this page of the guide to Genshin Impact you will read about expeditions - another in-game mechanic. You will find out what expeditions are, how to unlock them, and who is best to send on expeditions.
As the character develops and the storyline progresses (you must earn an AR rank of 14), you will be given the option to send your unlocked characters on expeditions. Characters are then unavailable until they have finished the task. Characters can be sent on expeditions of 4, 8, 12, or 20 hours. The longer the expedition, the better rewards you can get. Send on expeditions only and only characters you don't use, or have a special benefit for a given task. If you have characters you want to play with, use this option at a time when you're definitely not playing Genshin Impact. Time counts even when you're not online. You can cancel a character's expedition, but then you won't get any rewards for it, so plan carefully who you send there and for what time. You can only send two characters at once.