Ayuno is the first-person conjugation of the verb ayunar, meaning "to fast". This act is often associated with deep religious devotion and making a sacrifice for a higher purpose.
In "Mi Santa," Romeo Santos uses this powerful word to elevate his love to a spiritual level. By singing "Por ti ayuno y no me importa el tiempo" (For you I fast and time doesn't matter), he creates a dramatic metaphor, expressing his willingness to sacrifice his own basic needs out of complete devotion to his "saint".
Romeo Santos takes romantic obsession and turns it straight into a religion in "Mi Santa" (My Saint). Backed by the Spanish guitar master Tomatito, Romeo treats his lover not just like royalty, but as a holy idol. He declares, "Nadie puede comprender mi religión por tu querer, eres mi Diosa y yo hasta por ti muero" ("Nobody can understand my religion for your love, you are my Goddess and I'd even die for you"). He is willing to sell his soul to the devil, become an astronaut just to fly her to the moon, or transform into the jungle hero Tarzan to fight a lion if she asks. It is completely over the top, dramatic, and intensely devoted.
To really get his point across, Romeo borrows Catholic imagery and flips it onto his bedroom life. He sings that his holy sacrament took place in her bed ("Mi sacramento fue en tu cama"), calls her his bread and wine ("mi pan, mi vino"), and lights a literal prayer candle for her in his closet: "Enciendo una velita y en mi armario luego rezo una oración" ("I light a little candle and in my closet then I pray a prayer"). He explicitly tells her that she is his saint, admitting that the only two things he believes in are God and her.