This is the 403rd most frequent Portuguese word.
mais
"Mais" means "more" or "most" in English.
Here, 'mais' is used to request an additional amount of something, in this case, water.
Eu quero mais água.
I want more water.
In this sentence, 'mais' functions as a comparative, indicating a greater degree of a quality (interestingness).
Este livro é mais interessante do que aquele.
This book is more interesting than that one.
Here, 'mais' is used for expressing proportional relationships in a correlative conjunction structure.