Drooling
The drooling tag covers many anime art situations. The majority of its uses are humorous. Imagine someone knocked out at their school desk with saliva on their mouth or someone with stars in their eyes staring at a huge feast. Illustrators use this simple visual cue to show a character is relaxed, dreaming, or starving.
Adding some drool to the character makes it more interesting and messy. It ruins the perfect photo and makes them silly and friendly. This phrase has a different meaning in adult art. Spicy or ecchi fanart often depicts overwhelmed or flustered characters with drooling faces, heavy blushing, and dazed eyes. That little detail changes the picture's mood, from cute, innocent sleepiness to intense suggestiveness.
These wallpapers are popular because they show raw emotions that posed character portraits can't. A sleepy waifu napping at her desk is relatable, but spicier art is for a different person. This shows how a character's liquid can reveal their thoughts.