This is the 13681st most frequent Spanish word.
reír
"Reír" means "to laugh."
The word 'reír' is used here in its infinitive form to describe the act of laughing as an action.
Ellos no pueden dejar de reír con esa película tan divertida.
They cannot stop laughing at that very funny movie.
The word 'reí' is used in the first person singular preterite tense to refer to a completed past action where the speaker laughed.
Ayer me reí mucho durante la fiesta con los juegos.
Yesterday, I laughed a lot during the party with the games.
The word 'reír' is used here as an infinitive following 'hacer' to express causing others to perform the action of laughing.