This is the 13755th most frequent Korean word.
마시는
"마시는" means "drinking" or "that is drunk."
In this sentence, '마시는' describes the act of drinking the water, showing present tense and attributive adjective usage.
그녀는 마시는 물이 깨끗해야 한다고 했어요.
She said the water she drinks needs to be clean.
Here, '마시는' explains that the tea is being drunk currently, giving context to the situation.
마시는 차가 매우 뜨거워서 조심해야 해요.
The tea you are drinking is very hot, so you need to be careful.
In this instance, '마시는' connects the action of the children drinking the juice with the description of its source.