In short: a prepaid card is a card you first load with an amount (electronic money) and then spend only up to it; you cannot spend more than you loaded.
Simply put: It works like a voucher with a hard ceiling. It suits a limited budget — for a child, for online shopping or for travel. When the amount runs out, payments stop.
Example: You load €30 for the month onto a 13-year-old's card. They pay €12 for a book and €8 for the cinema. €10 is left, and they cannot spend more until the next top-up.
Watch out: ask about the top-up fee, the expiry date and whether the balance can be refunded. Money on a prepaid card issued by an e-money institution is not a deposit and is not covered by the deposit guarantee scheme.
Example: Bulgaria — funds received by an e-money institution are not a deposit (Payment Services and Payment Systems Act, Article 40(2)); registers of these institutions are kept by the BNB.
Check in your country — whether the issuer is a bank or an e-money institution, in your central bank's or financial supervisor's register.
Source: Regulation (EU) 2015/751, Article 2: https://eur-lex.europa.eu/eli/reg/2015/751/oj · BNB — registers under the Payment Services Act (in Bulgarian): https://www.bnb.bg/RegistersAndServices/RSPIPublicRegisters/index.htm