Nope, it has nothing to do with credit cards, although it also accepts them.
It is majorly used for debit cards, and similar in use to the famous Minitel in France.
You can use it to load pre-pay phones, or other kinds of rechargeable services, buy tickets for public transport and various kinds of shows, pay water, electricity, taxes, among other services.
There is now an app used to pay on shops via QR codes.
You can also pay online with one time cards, that are generated for a single transaction.
Outside Portugal it is a regular debit card.
When you access Multibanco with foreign cards, you can only withdraw money usually.