Compromiso means 'commitment' or 'engagement'. It's a powerful word in Spanish that signifies a deep promise and a serious step in a relationship.
In this romantic song, Fonseca playfully asks, "¿Dónde tengo que firmar este compromiso?" (Where do I have to sign this commitment?). He uses this word to show he's so completely in love that he's ready to make it official without a second thought, adding a charming and eager tone to his declaration.
This song is a massive, sweet declaration of love that lasts. Fonseca and Manuel Medrano aren't singing about a brand new crush; they're talking about a love that has already been through some stuff. They admit they've had to "sobrepasar tormentas" (get through storms) and that it hasn't always been perfect. But even after all that, the singer would repeat their entire story from the beginning.
The main idea is this beautiful promise to keep the romance alive forever. He wants to make his partner fall in love with him again and again, singing, "Quiero volver a enamorarte mil veces" (I want to make you fall in love with me a thousand times). He's not taking the relationship for granted. He wants to earn her love every single day, just to get a kiss. He even asks, "¿Dónde tengo que firmar este compromiso?" (Where do I have to sign for this commitment?), showing he is completely ready to make it official and permanent.