The word reías is the imperfect form of reír in the second person singular. There are other translations for this conjugation.
See the full reír conjugation

reír

Nunca vi a Colin tan feliz como cuando te reías.
Colin was never so happy as when you were laughing.
Papá me dijo que te reías, pero no le creí.
Daddy said you were laughing, but I didn't believe him.
Otros podrían pensar que te reías, pero no yo.
Other people might think you were, but not me.
Soñé que te reías de mí y querías hacerme daño.
I dreamt you were laughing at me, trying to hurt me.
Y te la cantaba a ti, y tu te reías.
And he sang to you, and you laughed.
Te reías de eso cuando tenías 4 años.
You would have laughed at that when you were four years old.
Sí, Srta. Te reías mucho con tu amigo esta noche.
You were laughing too loud with your friend this evening.
Te reías con él cuando bajabais del avión.
You were laughing with him when you got off.
Tú te reías de mi cuando te lo conté.
You were laughing at me when I was telling you.
Que te reías de mí a mis espaldas.
He said you were always laughing at me behind my back.
¿De qué te reías con Matthew, en la primera vuelta?
Why were you laughing with Matthew at the end of the first drive?
Te reías de todo lo que decía.
You were laughing at everything he said.
Te reías de la manera en que ella colgaba todo.
You used to laugh at the way she got everything on the drip.
Entonces ¿Por qué te reías con él?
Then why are you laughing with him?
Te reías todo el tiempo.
You were laughing all the time.
Tampoco te reías mucho con sus chistes.
You didn't laugh much at her jokes, either.
¿De qué te reías cuando entré?
What were you smirking at when I come in?
¿Por qué te reías de mí?
Why did you laugh at me?
Intentaba ver si te reías.
I was trying to check if you're laughing.
Por eso te reías en el patio.
That's why you were laughing at recess.
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