se me sale is an expressive phrase that literally translates to "it comes out of me". It's used in Spanish to describe an uncontrollable emotion that is bursting to get out.
In this song, the singer says "el corazón se me sale" (my heart is bursting out) and "una sonrisa se me sale" (a smile just escapes me) when he sees the person he loves. It’s a powerful and visual way to express a feeling so strong that it can't be contained inside.
This song is a full-on declaration of love, so intense that the singer feels like his heart is literally going to jump out of his chest. He says, "Te veo y el corazón se me sale, se me sale" (I see you and my heart jumps out, it jumps out). It's about a love that prioritizes the other person's happiness above everything else, promising to be there "Hasta en tus peores días" (Even on your worst days) and for as long as he lives.
He believes actions are more important than words, asking, "¿Pa' qué decirte que te amo? Si eso está de más / Lo quiero demostrar" (Why tell you I love you? If that's extra / I want to show it). He sees his partner as a divine gift, calling her "Un ángel caído del cielo" (An angel fallen from heaven) and admits he can't believe his luck. It's a sweet, totally devoted love song about feeling completely overwhelmed by affection.