This is the 14533rd most frequent German word.
lautstark
"Very loud" or "vocally expressive."
'Lautstark' is used as an adverb here to describe the manner in which the fans celebrated; it indicates intensity and volume.
Die Fans bejubelten lautstark den Sieg ihrer Mannschaft.
The fans loudly celebrated the victory of their team.
In this sentence, 'lautstark' again functions as an adverb, emphasizing the manner of expression as vocal and noticeable.
Er äußerte seine Meinung lautstark und unüberhörbar.
He expressed his opinion loudly and unmistakably.
Here, 'lautstark' is employed adverbially to describe the demanding action as being done in a loud and insistent manner.