Legión literally means "legion," like a vast number of soldiers. It's a powerful and dramatic word that you don't hear in songs every day.
In "Te Busco," Celia Cruz sings about "una legión de tus recuerdos" (a legion of your memories). She uses this epic metaphor to describe being overwhelmed by a massive army of memories, painting a vivid picture of her deep sense of loss and longing.
In this classic by the Queen of Salsa, Celia Cruz, she's searching for someone she has lost. It's a desperate, constant search. She looks to the sky, but her "estrellas no responden" [stars don't answer] to light the way. The search is so intense that the real world feels blurry and distant, like she's wrapped in a veil by the noise of the crowd.
The memory of the person she's looking for is fading away. She sings that waves are washing away her memories, stealing the shape of their face and "dejando arena en el silencio" [leaving sand in the silence]. She chases them everywhere, in dreams, in familiar places that now feel strange, and in shadows. But the person is gone, carried away by the wind "como un pañuelo viejo" [like an old handkerchief]. It's a powerful and sad image for a love that has been lost to time.