Detalles literally translates to "details", but in a romantic context like this song, it means so much more. It refers to the small, thoughtful gestures, surprises, or compliments that show affection and that you're thinking of someone.
In the song, the woman feels the passion is gone and wishes her partner would "hacerle detalles" (do these thoughtful things for her). It's not about grand, expensive gifts, but the simple, romantic acts that would make her feel loved and reignite their spark.
This song tells a classic story about a relationship where the couple seems to have forgotten how to connect. The man is a great provider, working late so his partner won't lack anything. He's responsible and pays all the bills, but he's so focused on work that he forgets the little things, like important dates. From her perspective, she feels the love is fading and "la pasión se congeló" (the passion has frozen).
What she wants isn't more money or material things. She just wants the romance back! The chorus is a list of what she's missing: "le hace falta un beso" (she needs a kiss) and "que le dé una rosa" (that he give her a rose). She wants him to make her feel "como cuando era su novia" (like when she was his girlfriend). The saddest part is the disconnect; at one point he asks, "¿Por qué está triste, mi reina?" (Why are you sad, my queen?), showing he has no idea what's wrong. He thinks providing is enough, but she just needs some love.