Mecí is the past tense of "mecer", a beautiful verb that means to rock, sway, or gently swing.
In a nostalgic line, Bad Bunny sings "me besaste y en el aire me mecí" (you kissed me and I swayed in the air). He uses this highly poetic word to vividly capture that magical, weightless sensation of floating you feel during a perfect first kiss.
This song is about that awful feeling when you know a relationship is over, but your partner is still pretending. Bad Bunny is basically begging his girlfriend to stop "Haciendo que me amas" (Pretending that you love me). He can tell everything has changed. The kisses don't taste the same, and even her voice is different when she says "I love you." He just wants her to be honest and tell him it's finished instead of dragging it out and making things worse.
He looks back on how great things used to be, remembering the day they met and even the intimate moments they shared "en la cocina y en el baño" (in the kitchen and the bathroom). Now, after all that time, they're strangers again. He's decided it's time to say goodbye, asking for his heart back "aunque sea en pedazos" (even if it's in pieces). The song ends with some really solid advice: don't hate each other after everything you've been through, and most importantly, "nunca te olvides de quién tú eres / Ni por hombres ni mujeres" (never forget who you are / not for men or for women).