Sassi means "stones" or "rocks". It's a simple yet evocative word that carries a lot of weight in the context of the song.
The artist repeatedly sings about throwing sassi alla finestra (stones at the window), a classic, somewhat old-fashioned gesture of trying to get someone's attention or signal a secret meeting. This imagery creates a sense of youthful rebellion and urgent desire, making the word memorable and central to the song's narrative.
"Sassi Alla Finestra" ("Stones At The Window") is about the push and pull of an early relationship, torn between wanting to see someone and being scared to.
The title image comes from an old, low-tech way of getting someone's attention at night: throwing a small stone at their window instead of calling out loud. The singer imagines doing exactly that, then losing his nerve and running off before she even comes outside. He would rather have a quiet, empty city ("la città spenta") than actually face her.
Around that central image are scenes of ordinary life getting in the way: family dinners, long days, and the awkwardness of sneaking out late just to be alone together. He also admits to going looking for her, half worried, half like a child looking for his mother, and to losing his temper instead of just listening.
By the end he is tired of throwing stones with nothing to show for it, but the chorus keeps repeating, showing he is stuck in the same loop of wanting to get close and then backing away.