Herbs are a crafting part in Dying Light.
They can be used for certain Boosters, as well as a number of other blueprints. Mainly ones that add a toxic effect to the player's weapons. They can also be used to craft Korek Machete 2.0 These can be found all over The Slums and Secter Zero, Usually By Water. They Are worth almost nothing and only use is for crafting as well as doing like making upgrades to the weapons you already have.
With luck, you can find it in any green area. WARNING! Not a good subtitute for toilet paper.