Acompleja comes from the verb acomplejarse, which means to feel insecure or to have complexes about something. It's a less common verb in everyday conversation, making it stand out.
In the song, Cris MJ sings, "Mi gatita no se acompleja," meaning "My girl doesn't feel insecure" or "My girl doesn't have complexes." It highlights her confidence and self-assuredness, which is an interesting character trait to feature in a song.
"Piso 13" is Cris MJ's play-by-play of a late-night hookup, dressed up as an address. He opens on a woman who answers to nobody, "Mi gatita no se acompleja" (My kitty's got no hang-ups), "De ninguna ella se deja" (She doesn't let anyone push her around), then gets straight to the point: "Hoy lo vamos a hacer en el piso 13" (Today we're going to do it on the 13th floor). The floor number never explains itself. It's just where the night happens.
The hook chants "En el 13" three times before every payoff line, which is the whole trick of the song: turn a number into something you can't stop repeating. Between rounds of that chant he circles back to compliments, "Su cuerpo lo probé y es toda natural" (I tasted her body and it's all natural), and to easy money, "el dinero llueve y llueve" (the money rains and rains), as proof the night is going his way. By the outro he's dropped any pretense of a bridge and is just repeating "Pa' que sepas" (so you know) over ad-libs until the track fades.