Gloria Trevi spends the first half of "Mentí" [I lied] walking an ex back through every memory they share. Did he remember the night they met, when her eyes "no sabían despegarse de ti" [couldn't tear themselves away from you]? The dress she stopped wearing for him? The nights she begged him, crying, not to leave? It all sounds like a woman still hung up on someone.
Then the hook flips the whole thing. "¿Recuerdas? Porque yo ya no me acuerdo" [Do you remember? Because I don't anymore], and then "Mentí, tonto, yo te mentí" [I lied, you fool, I lied to you]. The desperation, the forgiving, letting him humiliate her: she says she faked all of it. It's a great bit of pride, and also a little hard to believe, which is the fun of it. The best line is the denial in the chorus: "No estoy llorando, está lloviendo" [I'm not crying, it's raining], followed by "Aquí no pasa nada" [Nothing's going on here].
For learners, this song is a gift for the past tenses. You get preterite verbs like mentí, fingí and rogué for single actions, next to imperfect ones like iba obscureciendo [it was getting dark] and sonreías [you used to smile] for the background. Tonto is a very common, affectionate-to-insulting word for "silly" or "fool".