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In short
Adding money to a digital account or prepaid service to increase its balance.
In plain words
You put more money into a digital wallet, mobile phone credit, or transport card. This allows you to keep using the service without running out of funds.
A simple example
You use a banking app to move ten pounds from your savings into your prepaid travel card. Now you have enough credit to pay for your bus fare.
Why it matters
It allows people to use services without needing a full bank account or a monthly contract. It provides control over exactly how much money is spent at one time.
Easy to mix up
Do not confuse it with a refund, which is money being sent back to you.