Genshin Impact has a day and night system, which you adjust the time to freely in the game menu. You may use it for certain NPCs to appear or just for some beautiful screenshots.
Note: Skipping time won't affect the respawn time of items.
Some quests and puzzles require a certain daytime or timewindow to be accessable and completed. Remember to read the quest description or the notes in your backpack related to that quest!
Depending on the time, NPC can appear or disappear from location.
For example, most of the vendor NPC only appear on day and disappear on night althogh there are some vendor that do the oppsosite.
This is not only happening in city, but also in many location across Teyvat. If you come into correct time and place maybe you will come across NPC that sell you rare materials for low price or give you item for free!