Espejismo translates to "mirage". It's a captivating and somewhat uncommon word that evokes a sense of illusion and false hope.
In the song, Santiago Cruz sings, "Hoy te estrellaste contra un espejismo" (Today you crashed against a mirage). He uses this word to describe how the person he's addressing pursued something that seemed real and desirable but ultimately turned out to be an illusion, leading to disappointment and a broken life. This metaphorical use makes it a very impactful and memorable word.
Have you ever cared about an ex, even long after you broke up? Santiago Cruz explores this complex feeling in "Porque Yo Te Quise." The song tells the story of someone speaking to a former partner. He makes it clear that their relationship is a "caso cerrado"—a closed case—and they said goodbye a long time ago. He's not the same person she grew tired of, but her life also didn't turn out the way she'd hoped. She took a gamble on something new and it seems she lost.
The singer sees that his ex is now in a bad place. She was looking for something different but crashed into a "mirage" or espejismo, and now her life is falling to pieces. He wants to be clear: he's not trying to reopen old wounds or get back together. His concern isn't because he loves her now, but because he loved her in the past. It's because of that shared history that he offers his support, telling her, "hoy me pongo a tu lado / Por si quieres respirar" (today I'm by your side / In case you want to breathe).