This is the 702nd most frequent Catalan word.
riu
"Riu" means "river" in English.
In this sentence, 'riu' is a noun referring to a natural watercourse, a river.
El riu travessa la ciutat.
The river crosses the city.
Here, 'riu' is a verb, meaning 'he/she/it laughs', in present tense indicative mood.
Quan li expliques una broma divertida, ell sempre riu.
When you tell him a funny joke, he always laughs.
In this sentence, 'riu' again refers to the noun, a river, emphasizing its soothing sound.