Luto literally translates to "mourning," the period of deep sorrow, typically after a death. It's a somber and powerful word that you don't often find in a pop song.
In "Mutuo," Grupo Frontera uses it in a deeply poetic way. The singer creates a stark contrast with the line "Yo guardándote cariño y tú guardándome luto" (I was saving affection for you, and you were saving mourning for me). He's saying that while he was investing love in the relationship, his partner was already treating it as if it were dead and gone.
This song is all about that awful feeling when you realize a relationship was completely one-sided. Grupo Frontera and Carin Leon lay out the whole painful story of a guy who thought he was in a loving, mutual partnership, only to find out he was the only one invested. He sings, “Pensé que era mutuo” [I thought it was mutual], but the reality was so different. While he was “guardándote cariño” [saving affection for you], she was “guardándome luto” [saving mourning for me]. That line is just brutal, isn't it? He was dreaming of forever, and she was already planning the funeral for their relationship.
The lyrics get right to the heart of the confusion and betrayal. The singer asks questions that anyone who's been led on will understand: “Si no querías nada serio, ¿pa' qué ilusionas?” [If you didn't want anything serious, why get my hopes up?]. The worst part is the public humiliation. He was telling people they were together, while she was telling everyone “que andas sola” [that you're single]. In the end, he admits that even if he wanted to forgive her, some things are just too much. He says, “hay cosas que no se perdonan” [there are things that can't be forgiven], and you can really feel that finality.