Años luz means "light years", the astronomical unit for the distance light travels in a year.
Matisse use it in the title line "mil años luz" (a thousand light years) to say that no amount of time or distance will stop him loving her. It is a science word you rarely hear in a romantic ballad, which makes the promise feel cosmic in scale.
This is Matisse at his most openly smitten. Every time he looks at her, "me siento vivo" (I feel alive), and his heart beats "el doble de latidos" (double the beats). The world is a strange place, but it makes sense "solo contigo" (only with you). He even wants it to be winter all year round, purely as an excuse "para abrazarnos" (to hold each other). Cheesy? Completely. Charming? Also yes.
The chorus is pure gratitude: "Qué buena suerte tenerte" (What good luck to have you), followed by "Que nadie me despierte" (Don't let anyone wake me up), as if this love is a dream he refuses to leave. The title comes from the big promise: even if life passes and "mil años luz" (a thousand light years) go by, "yo siempre voy a amarte" (I'm always going to love you). Light years measure distance, not time, but in a love song nobody is checking.
One line worth stealing: "se me quitó lo triste" (the sadness went away from me). That little se me construction is how Spanish speakers describe something happening to them rather than something they did, and it turns up everywhere in everyday speech.