Summer Herb is a Quest related item necessary for obtaining a Formation Manual.
Obtainment[]
Acquired by interacting with the 
Caterpillar Fungus (Wormwrought Valley) during the Summer Season[1], as well as by purchasing from Sects or the Merchant.
Uses[]
Visiting Celestial Pool while having both Summer Herb and Winter Worm buffs active drops the Formation Manual "Polarized Diagrams of Mt. Zhongyuan"[1].