What does "servizio incluso" mean on my bill?

Service is already included, so add nothing further. It's a percentage added to the total — most common in tourist areas, on larger groups, and in Venice and Rome — and it functions as the tip. If you see it, you're done: rounding up a euro or two is a kind gesture and nothing more is expected. Don't confuse it with coperto, the small per-person cover charge, which is not service and goes to the restaurant. If a bill has both servizio and coperto, that's legal but it's a sign you're somewhere that prices for visitors.