Porteiro means "doorman" or "concierge". It's an interesting word because it's central to the song's humorous and somewhat desperate plot.
The protagonist takes a job as a doorman in his ex-girlfriend's building just to keep an eye on her and control who goes in and out of her apartment. This unusual and slightly obsessive act makes the word memorable and key to understanding the song's narrative.
Have you ever done something extreme to win someone back after a breakup? In "Apartamento 119," Henrique Casttro tells a hilarious story about doing just that. After his girlfriend gets fed up with his bad habits, like coming home late, she kicks him out and even throws his clothes out the window!
But instead of leaving, he spots a job opening for a porteiro (doorman) in her very own building and takes the job immediately. His motive isn't the low salary; it's to keep an eye on her constantly. He now playfully boasts that he's the one in control, deciding who gets to go up to her apartment, number 119. He's even working overtime just to watch her, hoping she'll eventually take him back.