This is the 8571st most frequent Spanish word.
rápidos
"Rápidos" means "rapids" (fast-flowing sections of a river).
In this context, 'rápidos' serves as a noun referring to the turbulent sections of a river.
El río tiene rápidos peligrosos.
The river has dangerous rapids.
Here 'rápidos' is used as an adjective describing the characteristic of the nouns 'los carros'.
Los carros rápidos ganaron la carrera.
The fast cars won the race.
'Rápidos' is colloquially used to describe the manner of completing an action, working adverbially. However, the grammatically correct adverb here would be 'rápidamente' for formal usage.