Se marrer is a colloquial synonym of rire, “to laugh” or “to have fun”.
Je me suis marré pendant tout le film.
I laughed during the entire movie.
Ça te fait marrer, le malheur des autres?
Do the misfortunes of others make you laugh?
A related word is marrant, “funny”.
Ton frère, c’est un mec marrant.
Your brother is a fun guy.
Another colloquial word for “to laugh” or “to have fun” is rigoler, and another word for “funny” is rigolo (feminine form: rigolote). However, these words (especially rigolo) sound a little childish.
J’ai bien rigolé quand j’ai vu la tronche qu’elle faisait.
I had a good laugh when I saw her face.
Listen to the examples: