Sendero translates to "path" or "trail". While it might seem simple, it's a more poetic and less common word than camino (road or path), making it stand out in the lyrics.
In the song, Ana Gabriel implores the moon to "Ilumínale con tu luz su sendero" (Illuminate his path with your light). She's asking the moon to guide her beloved, emphasizing a journey or direction in life rather than just a physical route. This metaphorical use adds depth and emotion to her plea.
Ana Gabriel sings to the moon because the person she loves is gone and the moon is the only thing still watching over him. “Luna, tú que lo ves, dile cuánto le amo” (Moon, you who see him, tell him how much I love him) opens the song, and by the second verse the plea sharpens into “dile cuánto lo extraño” (tell him how much I miss him). She is not talking to him directly. She is talking to the one witness they still share.
The moon becomes a messenger for everything she cannot say herself: “Aconséjale por favor, de que vuelva convéncelo te ruego” (advise him please, that he comes back, convince him, I beg you). By the end she is not even asking anymore, just naming the ache plainly: “Y dile que lo quiero” (and tell him that I love him). It is a bolero built entirely on that one image, a woman staring up at the same sky he is under, hoping distance is not the same as gone.