This is the 247th most frequent Croatian word.
brzo
Quickly.
In this sentence, 'brzo' is an adverb describing the manner of running, showcasing speed.
Trčim brzo do kuće.
I run quickly to the house.
Here, 'brzo' describes the action of setting, emphasizing that it happens at a fast pace.
Brzo sunce zalazi iza planine.
The sun quickly sets behind the mountain.
In this context, 'brzo' is used as an adverb to describe the speed of the car's motion.