Llanto means "weeping" or the act of crying. It's a poetic noun that captures a deep sense of sorrow, often used in literature and music instead of the more common verb "llorar" (to cry).
In the song, The Marías sing "Entre mis labios, a todo llanto" (Between my lips, in a flood of tears). This beautiful and melancholic phrase paints a vivid picture of someone crying their heart out, making llanto a memorable word to express profound sadness.
This song is a beautiful, sad daydream about missing someone. The singer is haunted by memories, both good and bad. Everything seems to trigger a thought of the person they're separated from: "El frío, la noche / Siempre me acuerdo de ti" (The cold, the night / I always remember you) and even "Miles de canciones" (Thousands of songs).
The central question is simple and painful: "¿Por qué estoy lejos de ti?" (Why am I far from you?). The singer feels completely overwhelmed by their feelings, asking "¿Por qué te sigo queriendo? / Ya no puedo más" (Why do I keep loving you? / I can't anymore). They even reference another famously sad song, asking why they're still singing Selena's "No Me Queda Más." It's a confession of being totally lost in the feeling of heartbreak, remembering intimate moments like sleeping on their lover's chest and pleading, "No te olvides de mí" (Don't forget me).