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.
Ever been completely convinced you and your crush were on the same page, only to find out you were reading entirely different books? That is the exact gut punch Grupo Frontera and Carin Leon sing about in "Mutuo". The title means "Mutual," but the whole track is a sharp confrontation about how one-sided the relationship actually was. The singer was out here dreaming of "un pa' siempre juntos" ("a forever together"), while the other person was already cutting them out of their life completely.
The lyrics hit hard with clever wordplay in Spanish. Lines like "Yo poniéndole amor y tú poniéndole punto" ("Me putting in love and you putting an end to it") play on punctuation, where putting a punto literally means ending the sentence or calling it quits. The ultimate insult comes when the singer tells everyone they are a couple ("Yo diciendo que andaba contigo"), while the other person goes around claiming they are completely single ("Tú diciendo por ahí que andas sola"). It is blunt, painful, and asks the ultimate post-breakup question: if you never wanted anything serious, why lead someone on in the first place?