Otoño translates to "autumn" or "fall". While it's a season, its use here is particularly poignant and less common than other seasonal references in love songs.
In the lyrics, "Se me fue el otoño jurando mentiras" (Autumn left me swearing lies), the word otoño symbolizes a period of time, perhaps a season of the relationship, that has passed. It evokes a sense of fading beauty and the melancholic passage of time, making it a more evocative choice than a simple time marker.
Diego Torres sings this from the far side of a breakup he caused himself. The first verse is an admission, not a complaint: "Me hice el sordo y ciego y no te quise hablar" (I acted deaf and blind and didn't want to talk to you), while she was telling him things "en silencio" (in silence) that he chose not to hear. The seasons carry the damage — an autumn that walked out "jurando mentiras" (swearing lies), and then "Me entregué al invierno y a la soledad" (I gave myself to winter and to solitude).
The chorus is what a late apology actually costs. "Me quedo sin fe, me muero de sed" (I'm left without faith, I'm dying of thirst), and then the title arrives as plain arithmetic rather than an image: "No alcanzan las flores / Para pedir perdón" (The flowers aren't enough / to ask forgiveness). Flowers are the standard apology, and he is saying they don't cover what he owes. By the last verse he has stopped bargaining altogether: "Soy sólo una sombra sin rumbo y sin paz" (I'm just a shadow without direction and peace).