Cartão vermelho literally translates to "red card," as in soccer. It's an interesting phrase because it immediately brings to mind the imagery of a soccer match.
In this song, the phrase is used metaphorically to describe a definitive rejection or expulsion from one's heart, comparing a romantic breakup to a player being sent off the field. This clever use of a common sports term in a romantic context makes it memorable and engaging.
Have you ever felt like love should have rules, just like a game? In "Cartão Vermelho," which means "Red Card," the singer uses the world of soccer to describe a painful relationship. He feels like his lover has committed a "criminal tackle" on his heart, leaving him hurt and knocked down. It's not a fair game, and he wishes he could be the referee.
Just like in a soccer match, the singer wants to give this person a red card. This would mean kicking them out of the "game" of his heart for good. He sings that if he could issue a red card for every foul this person commits, they wouldn't be able to "play" with his feelings anymore. The song captures the frustration of being in a one-sided love where the other person only shows up when they feel like it, and you wish you had the power to just call "game over."