Hola Lilou

Reír and reírse both mean "to laugh", but we use it more as a pronominal verb with "se". You can say: "El niño está riendo" or "El niño se está riendo" (The boy is laughing)

However if we mention what the person is laughing about, you need "reírse":

"El niño se está riendo de su hermana" (The boy is laughing about his sister)

"Me río de ese chiste" (I am laughing about that joke)

"Nos reímos de la película" (We are laughing about the film)

Notice that we use "de..." to say "about..."

Hope this helps

Inma