Currency conversion is the exchange of one currency for another at the rate applied by a bank or service.
In plain words
Conversion happens whenever the account currency differs from the payment currency: you paid in euro with a hryvnia card, moved dollars to a hryvnia card, or changed cash at an exchange office. The rate usually differs from the official NBU rate: the bank can build the difference (the spread) and a fee into the rate. So compare the final amount you will receive or pay, not only “0% commission”.
Example. You paid EUR 10 for a service with a hryvnia card. The payment system converts the amount into the account currency at its own rate and the bank may add its markup: the final hryvnia amount depends on both.