"Jamás, jamás" (Never, never) is one long, loud promise. It opens like a live show, with Café Quijano calling out "Mis amigos" (my friends) and bringing in Venezuelan singer Carlos Baute, and then it goes straight into missing someone: "si me faltas, me falta el alma" (if I'm missing you, I'm missing my soul). He wakes up every morning wanting her caresses and kisses and says flat out: "No puedo vivir sin ti" (I can't live without you).
The hook is where you learn the most Spanish. "No te voy a dejar jamás" (I'm never going to leave you) and "Ni te voy a fallar jamás" (nor am I ever going to let you down) use ir a + infinitive, the easiest way to talk about the future, and stack the word jamás three times for emphasis. Fallar a alguien means to let someone down, not just "to fail". There is also the very Spanish double negative, "no te voy a dejar jamás", where no and jamás work together instead of cancelling out.
It is sweet and a bit over the top, which is the point. Near the end they even push it to "Son siete vidas y un jamás" (it's seven lives and a never), and then someone shouts "Díselo Manolo" (tell her, Manolo), a playful nudge to Manolo Quijano to sing the chorus again.