Virtud means "virtue," a word describing a high moral standard or a particularly good quality. It's a beautiful and somewhat formal word that you don't hear in every song.
In this heartfelt track, the singer praises his partner with the line, "Ser muy buena es tu virtud" (Being very good is your virtue). Using virtud elevates the compliment, suggesting this goodness is a core, admirable part of her character, making his plea for her to stay even more powerful.
This song is a powerful and direct plea from someone who is completely devoted to their partner. The singer isn't playing it cool at all. He lays it all out, saying he needs this person with him always: "Donde esté hoy y siempre / Yo te quiero conmigo" (Wherever I am, today and always / I want you with me). He admits his total dependence on them for care and guidance, telling them, "Tú me sabes bien cuidar" (You know how to take good care of me).
He feels so much gratitude for the happiness this person brings that he asks how he could ever repay them. His answer is pretty intense. He offers up his entire life, singing, "Solamente con mi vida / Ten mi vida, te la doy" (Only with my life / Take my life, I give it to you). All of this builds up to the song's main point, a desperate request repeated over and over: "Pero no me dejes nunca, nunca, nunca" (But don't ever, ever, ever leave me).