Like a lot of cities and travel destinations, it’s possible to purchase a discount card in Naples that enables you to have free/discounted access to various historical and cultural sites around the city. Sometimes these things are worth it, sometimes they are not.
In Naples however, I would strongly advise picking up a Campania ArteCard swiftly after arriving in the city. It can be purchased at the airport, and at the ticket office of various tourist sites (at the archaeological museum, at Pompeii, etc).
The cards allow free access to some sites, free use of public transportation, and reduced entry at additional sites. Prices start from €12 for those under 25, and €21 for those over 25. Either way, it will save you a fair bit.