Lucero literally translates to "bright star" or "morning star." It's a beautiful and somewhat poetic word that isn't as common as "estrella" (star).
In the song, the artist uses lucero to describe his beloved: "Eres mi vida, eres mi lucero" (You are my life, you are my bright star). It conveys a sense of someone being a guiding light, a source of hope and beauty, making it a very endearing term of endearment.
In "Mar y Tierra," Santa Fe Klan tells the story of a long and difficult search for true love. He describes a past filled with mistakes, loneliness, and pain, singing that he was tired of "walking without direction." He traveled everywhere, or by "sea and land" (mar y tierra), looking for the right person to heal his wounds and bring light back into his life.
After finally finding his special someone, the song becomes a heartfelt promise of devotion. He swears he will not fail her and will be with her through good times and bad. He offers total protection and care, promising to give her anything she wants. The journey was tough, but finding this love has given him a new purpose and a reason to leave his past behind for good.