Mamey literally refers to a sweet tropical fruit, but in Puerto Rican slang, saying something is "un mamey" means it is incredibly easy, just like the English idiom "a piece of cake".
In the song, the singer uses the phrase "Esto es fácil, esto es un mamey" to convince a shy girl that dropping her serious attitude and dancing is completely effortless. It is a fun, highly memorable slang term that adds authentic Caribbean flavor to this energetic track.
This song is a high-energy invitation to let go of your inhibitions and just dance. Calle 13 is talking directly to someone he sees as too serious, calling her a "Señorita intelectual" [Miss intellectual] who needs to change her "cara de seria" [serious face]. He wants her to stop pretending she doesn't like reggaetón, even if her favorite bands are Coldplay or Green Day. The song argues that the beat is irresistible, saying "el reggaetón se te mete por los intestino'" [reggaetón gets into you through your intestines] and brings out a wilder, more primal side.
The chorus is a series of commands telling this person to break free. "Atrévete-te-te, salte del clóset" [Dare yourself, come out of the closet] isn't about sexuality, but about coming out of your shell and showing your true, fun-loving self. He tells her to get energetic, or "ponte hyper" [get hyper], and embrace her inner "callejera, Street Fighter" [street girl, Street Fighter]. It's a fun, slightly provocative anthem about not caring what anyone thinks and letting the music take control.