This is the 9074th most frequent Spanish word.
viértele
"Viértele" means "pour it to him/her/it" in English.
Here, "viértele" is a command form, derived from "verter" (to pour) with an indirect object pronoun (le) referring to 'the plant', instructing someone to pour something for it.
Viértele agua a la planta para que crezca.
Pour water onto the plant so it grows.
In this case, "viértele" structures a suggestion for adding a liquid to modify the consistency of something, incorporating 'le' to denote the target (the broth).
Si el caldo está espeso, viértele un poco más de agua.
If the broth is thick, pour a bit more water into it.
Here, "viértele" serves to communicate the action of transferring a substance as directed, with 'le' indicating the recipient of the contents.