It's a paper revenue stamp you buy at a tobacconist and stick on official documents to pay the imposta di bollo. Common amounts are 16 euros and 2 euros.
Many comune-issued certificates and some application forms need a marca da bollo. Without it, the document is incomplete and the counter can refuse to accept it.
"Before going to the comune of Pordenone for a stato di famiglia in carta bollata, Andrija stopped at a tabaccaio to buy a marca da bollo da 16 euro and stuck it on the application."
More and more documents accept a virtual bollo paid online. Always ask whether you need the paper stamp or if you can pay the duty digitally.
Official source
Agenzia delle Entrate
Italian phrases with audio for the conversation this paperwork leads to.
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