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
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