The word reíais is the imperfect form of reír in the second person plural.
See the full reír conjugation


¿De qué os reíais el otro día?
What was you and her laughing at the other day?
¿De eso os reíais cuando entré?
Is that what you were laughing at when I came in?
¿Os reíais de mí juntos?
Did you laugh at me together?
Solo os reíais de mí.
You just laugh at me.
¿De eso os reíais los dos?
That's why you two were laughing?
¿De qué os reíais?
What were you laughing about?
¿De quién os reíais?
At whom are you laughing?
¿Y os reíais tanto?
Did you laugh about it?
¿Por eso os reíais? ¿De mí?
That's why you two were laughing?
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