Public Digital Identity System
Plain English
Your digital identity for dealing with Italian public services online. It's a username, password, and second factor that you get from an approved provider like Poste, Aruba, or InfoCert. Once active, the same login works across every public-sector website.
Why it matters
You'll need it to file taxes, check INPS contributions, book vaccinations, apply for the AUU, request your tessera sanitaria, and dozens of other day-to-day things. Most online portals will not even open without one.
Real example
"Yuki signed up for SPID through Poste Italiane. She booked a 10-minute identity-verification appointment at her local post office in Trieste with her passport and codice fiscale, and her account was live the same evening."
Common confusion
There are three security levels (1, 2, and 3) and most government portals want level 2, which means using the app or an SMS code at every login. Level 1 (just password) almost never works for anything important.
Official source
AgID (Agenzia per l'Italia Digitale)