Faltas comes from the verb faltar, which means 'to lack', 'to be missing', or 'to be absent'. While it's a common verb, its use here is particularly poignant and dramatic.
In the context of the song, Leo Dan sings, "Si un día me faltas tú, Que Dios me ayude a morir." This translates to "If one day you are missing from me (or if I lack you), May God help me to die." It's a powerful expression of his absolute dependence on Mary for his happiness, making the word faltas carry a profound emotional weight that is both memorable and unique to the song's intense declaration of love.
Leo Dan's "Mary Es Mi Amor" is a heartfelt declaration of love and devotion. He tells us that Mary is his love, and it's only with her that he experiences happiness. He's so sure of this, he says, "Yo sé que nunca a nadie más podría amar / Porque la quiero de verdad" [I know I could never love anyone else / Because I truly love her]. He even asks Mary to hold his hand so they can continue on together, because he's already so happy with her.
This song takes a dramatic turn when Leo Dan considers a life without Mary. He states, "Si un día me faltas tú / Que Dios me ayude a morir" [If one day you are missing from my life / May God help me to die]. That's a pretty intense way to say he can't imagine happiness without her! He repeats that he won't be happy in this life without her, emphasizing, "Sin ti, Mary / Sin ti, Mary / Si estoy viviendo por ti" [Without you, Mary / Without you, Mary / If I am living for you]. It's a powerful and direct expression of how central Mary is to his existence.