Are you asking about a rooting hormone? It is used to dust or coat the lower part of a plant cutting before it is placed in damp soil to encourage it to produce roots and become a new plant. See these web pages.  
https://www.purdue.edu/hla/sites/yardandgarden/extpub/new-plants-from-cuttings-text-only/
https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/visual-guides/rooting-cuttings-in-water#:~:text=Place%20cutting%20in%20water,Pre%2Dmoisten%20some%20potting%20soil
Placing a plant cutting in water is used to encourage the creation of new roots. The cutting and roots are then placed in soil to create a new potted plant. Hormones are not used in water, just soil. The list of plants that may be propagated in either method is extensive. Look up the plant you want to propagate online or in a reference book to decide which method to use.