Descifrar means "to decipher" or "to decode." It's a more intriguing and less common verb than simply "to understand" or "to know."
In the song, the singer wants to "descifrar esa dulce boca" (decipher that sweet mouth). This implies a desire to deeply understand and explore the person's kisses and affections, adding a layer of mystery and intimacy to the romantic lyrics.
Belanova build this one around a single request, repeated until it turns into a vow: "Bésame una y otra vez" (kiss me again and again). There's no backstory and no complication here, just a couple who found each other and can't stop saying so. The hook says it outright: "Eres mío, sólo mío, yo soy tuya" (you're mine, only mine, I'm yours), traded back and forth like a promise neither of them is tired of making.
What keeps it from feeling flat is the specific, small-scale shape of the happiness. She doesn't ask for anything grand, she wants to dance: "Solo quiero bailar bajo el cielo azul" (I just want to dance under the blue sky). And the line that explains why the whole song stays this light, "No siento miedo ni vacío si yo te puedo besar" (I don't feel fear or emptiness if I can kiss you), treats a kiss as an actual answer to loneliness rather than a metaphor for one. It's a love song with nothing to prove, which is rarer than it sounds.