"Error" opens the morning after. "Hoy me llamó arrepentida" (today she called me, full of regret), asking why he had to mess with her head. His answer is basically: you weren't saying that last night. The hook is the whole argument in four lines: "Dices que lo de nosotros es un error, pero en la cama se nos pasan las horas" (you say what we have is a mistake, but in bed the hours fly by).
Lo de nosotros is a handy phrase to learn: literally "the thing of ours", it means "what we have" or "this thing between us". And "Si tú quieres que me vaya, pues me voy" (if you want me to leave, then I'll leave) is followed straight away by a cocky "pero para qué" (but what for?), because he knows she will call again.
Lalo Ebratt's verse is the bluntest part of the song and gets very explicit, so expect some lines you would not say to your teacher. Later the tone softens a little: she has a boyfriend who is always ausente (absent), and the singer tells her "No seas tan cruel contigo" (don't be so hard on yourself), she was only looking for company. Whether that is kindness or a smooth excuse is up to you.