Medallo is a playful Colombian nickname for Medellín, the city celebrated in the line “la vista de Medallo” (“the view of Medellín”). It is a memorable regional expression that gives the song a strong sense of place and introduces learners to the slang and culture of Colombia.
Kevin Roldán is replaying last night, and he is not being subtle about it. The chorus says "Anoche cuando te besé me sentí en el cielo" (last night when I kissed you I felt like I was in heaven), and then gets a lot more explicit fast. There's also a jealous edge: "Ese cabrón te tenía, mami, fría como un hielo" (that bastard had you, baby, cold as ice), so he's bragging that her ex couldn't make her feel anything and he can.
This is a great song for Colombian slang. Qué chimba means "how awesome", mela is "cool", and Medallo is the nickname for Medellín, his city. El tussi is a party drug popular in Colombia, so "el tussi amarillo nos tiene en Marte" (the yellow tusi has us on Mars) is about being very high. Seti and seteao' mean "on point". The ex se rayó (lost it) and still texts her 'I love you' on WhatsApp, which Kevin answers with a blunt English "f*ck you". And in case you forgot who is singing, she says the best is "Kevin", followed by a sound effect: "bofff".