This is the 1152nd most frequent Portuguese word.
muito
"Much," "very," or "a lot."
In this sentence, 'muito' is used as an adverb to intensify the adjective 'feliz' (happy), meaning 'very'.
Ela está muito feliz hoje.
She is very happy today.
Here, 'muito' functions as a quantifier indicating a large amount of 'trabalho' (work), meaning 'a lot'.
Eu tenho muito trabalho para fazer.
I have a lot of work to do.
'Muito' in this context is an adverb modifying the verb 'estudamos' (studied), equivalent to 'a lot'.