Zarri is a slang term, particularly used in some parts of Italy, to refer to young, often rebellious individuals, sometimes associated with a rough or delinquent lifestyle. It's not a common word in everyday Italian, making it interesting and unique to this song's context.
In "15 Piani," Sfera Ebbasta uses zarri to paint a vivid picture of his youth and the environment he grew up in, specifically mentioning "quindici zarri" (fifteen thugs/delinquents). This word helps to convey the raw reality and struggles of his past, adding a layer of authenticity to his storytelling.
Everything in this song hangs off one number. Fifteen is the floor count of a Milan council block, the age you stop being a child there, and the weight in a bag: "Quindici in giro, quindici TMAX, quindici grammi" (Fifteen of us out there, fifteen TMAXes, fifteen grams). The chorus keeps landing on "Quindici in quindici metri quadrati, ma siamo riusciti a salvarci" (Fifteen of us in fifteen square metres, but we managed to save ourselves), and the ma (but) sitting in the middle of it does most of the work. Before it the verse climbs the stairs a floor at a time, "Un piano, due piani, tre piani, poi quattro" (One floor, two floors, three floors, then four), which is a broken lift or a career depending on how you hear it.
The second verse pushes the pun until it hurts, stacking quindici (fifteen) against quindi (so) until arrests, school desks and days of holiday all sound like the same word. What surrounds it is more bruised than boastful. "Ti amano finché sei povero" (They love you as long as you are poor) sits a few bars from a helicopter passing over the old Aler housing blocks, and "Non ho vinto al Lotto, però dopo anni che lotto" (I did not win the lottery, but after years of fighting) is the sharpest line here, because lotto is the lottery and also I fight. Even the flex is bookkeeping: what four of them once spent on clothes in a year is now the dry cleaning bill.