It's an order from the Questore telling you to leave a specific municipality (and not return for a set period). It's used as a public-order measure, not a deportation tool, and applies to anyone whether Italian or foreign.
It can be issued without a conviction and is appealable in court. For foreign citizens, breaking it can complicate residence permit decisions later.
Foglio di via is not the same as espulsione. The first bans you from a town for a while. The second removes you from the country.
Official source
Polizia di Stato
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