Añorando comes from the verb añorar, which means to long for, to yearn for, or to miss deeply. It's a more intense and poetic way to express missing someone or something than simply saying extrañar.
In the context of
Twelve lines, and most of them are the same three sentences said again — that repetition is the song. It opens with someone marking time instead of living it: "Estaré contando el tiempo en mí" (I'll be counting the time inside me), "Contando hasta morir" (counting until I die). The counting never arrives anywhere, and between those measured beats sits the line that admits what it is for — "Qué falta me haces aquí" (how I miss you here).
The middle lifts into something weightless, "Estaré volando entre niebla y el mar" (I'll be flying between fog and the sea) and "Volando entre dioses y paz" (flying between gods and peace), then drops on "Qué rabia que ya tú no estás" (what rage that you're gone). Rabia is anger, not sorrow, and it is the sharpest word here. After it the song gives up on new phrasing and loops: "Llorando, me la paso llorando" (crying, I spend my time crying), "Me la paso añorando" (I spend it longing), "Me la paso en ti" (I spend it on you). Me la paso is ordinary Spanish for how you pass your days; by the last line it has turned a person into the place they get spent.