Ajena is a fascinating adjective that translates to 'belonging to someone else', a powerful concept that lacks a direct single-word equivalent in English.
In regional Mexican music, referring to a person as ajena specifically means they are already in a committed relationship with another partner. Christian Nodal uses this word to confess his intense attraction to a woman who is taken, perfectly capturing the forbidden drama and temptation at the heart of the song.
This song is a dramatic story about a secret love affair. Christian Nodal and Peso Pluma sing from the perspective of a man who is completely obsessed with a woman who is already in a relationship. He knows it's wrong, admitting she's his karma and that he's willing to risk it all, even if it means his soul burns in hell: "Si en el infierno mi alma se quema / Valdrá la pena" (If my soul burns in hell / It will be worth it).
The whole song is a direct challenge to her current partner. The singers argue that while her boyfriend might be the 'good guy' who buys her nice things, he can't provide the passion she really wants. They make their point very directly with the line: "Sí, él te compra esos vestidos, pero yo te desvestí" (Yes, he buys you those dresses, but I undressed you). They believe her life with her partner has no real excitement, "aunque tenga la intención" (even if he has the intention), and that she secretly misses the thrill they share.