Sfiga is an informal, somewhat colloquial Italian word for 'bad luck' or 'jinx'. It's more vivid and impactful than the more common 'sfortuna'.
In the song, Ligabue sings, "Che la fortuna è cieca e che la sfiga ha dieci decimi da sempre" (That fortune is blind and that bad luck has perfect vision, as always). This line uses 'sfiga' to create a memorable contrast, suggesting that while good luck might be random, bad luck is always sharply focused and present. It's a word that adds a touch of gritty realism and humor to the lyrics.
"Si dice che" means "they say that" — and that phrase is the whole song. Ligabue strings together the sayings, proverbs, and folk wisdom everyone repeats without really checking: heaven has plenty of room and few people in it, hell might actually be fun, whoever breaks something ends up making someone else pay, luck is blind, and bad luck always finds you. He lists them side by side and lets them contradict each other — "everything is its opposite" is the line he keeps coming back to.
Between the sayings, he pulls back to admit he doesn't know any better than anyone else. He says he's just speaking for himself, that he'll wander off and see what's out there and catch up with you later. It's less a statement of belief than a shrug at how much of what we "know" is just something we heard somewhere.
By the end he's dreaming about winning big or landing in some kind of trouble, and admitting flat out: "sai che non so" — you know what, I don't know. The song never resolves any of its proverbs. It just leaves them stacked up against each other, which is the point — it's a song about how much of everyday wisdom is really just hearsay.