"Sinal vermelho" means a red traffic light, and here it's the signal to stop a relationship that has run its course. "Sinal vermelho é hora de parar" (Red light, it's time to stop), she sings, so they can think about everything that happened. There's "não tem mais clima" (no more mood, no spark left) to carry on, and they've been "remando contra a maré" (rowing against the tide), a nice idiom for fighting a losing battle.
What I like is that she doesn't blame just one person. "Nós somos sempre reféns" (We are always hostages) of this love, and "julgamos donos da verdade, nunca abrimos mão" (we thought we owned the truth, we never gave in). Both were stubborn. "Começou tudo errado e isso não conserta mais" (It all started wrong and it can't be fixed anymore). It's a breakup song that's sad but surprisingly calm and mature.