Ingrávidas translates to "weightless" or "gravitationless". It's a beautiful and somewhat uncommon word that evokes a sense of lightness and freedom.
In the song, Fito Páez sings, "Todas las palabras son ingrávidas palabras" (All words are weightless words). He uses this word to suggest that in the face of profound love and connection, ordinary words lose their usual weight and significance, becoming light and perhaps even insufficient to express the depth of his feelings. This poetic usage makes it a truly memorable and evocative word.
Fito Páez wrote this one as a straight declaration, no metaphors standing between him and the person he's singing to. He opens with "Ella se divide en dos" ("She divides herself in two"), then finishes the thought in the next line: "la sombra y la luz del mundo" ("the shadow and the light of the world"). She holds both halves of everything at once, and that's exactly what pulls him in.
The chorus turns on one line, repeated with a small but important shift. First it's "Entonces, yo siento que me da su vida" ("Then, I feel that she gives me her life"), and later it becomes "Entonces, yo siento que me das tu vida" ("Then, I feel that you give me your life"). The song moves from watching her give herself to speaking straight to her. In the bridge, "Todas las palabras son ingrávidas palabras" ("All the words are weightless words") sums up the whole feeling: nothing said in that room needs to carry any weight to matter. It's a love song with no doubt in it at all.