Resplandor translates to "radiance" or "glow". It's a beautiful and evocative word that paints a picture of light and brilliance.
In the song, the narrator sings, "Seguramente se me note el resplandor de una ilusión" (Surely the radiance of an illusion will be noticed in me). This suggests a deep, internal glow that comes from hope and new feelings, making it a poetic and memorable choice.
Alejandro Sanz builds "Siempre Es De Noche" around one image he keeps returning to: night as the place he lives in whenever the person he loves isn't there. The opening verse watches a sunset together — "Cuéntame como va cayendo el sol" ("Tell me how the sun is setting") — and that ordinary moment becomes the whole point of the chorus: "Porque a tu lado puedo olvidar / Que para mí siempre es de noche" ("Because by your side I can forget / That for me it is always night").
The second half turns into someone else's story, told in the third person: a soul that has moved past its own darkness — "No volverá a perderse en la noche / Porque su alma hoy brilla con más fuerza" ("He won't get lost in the night again / Because his soul shines brighter today") — "Que un millón de soles" ("Than a million suns"). The song closes on the line it has been building toward the whole time, repeated three times over the outro: "Qué no daría yo por contemplarte, aunque fuera un sólo instante" ("What wouldn't I give to contemplate you, even if it were just for a moment").