Dynamic currency conversion (DCC) allows cardholders to pay for a product or a service in a foreign country using the home currency of the card instead of the local currency. The service provider will do a currency conversion applying their own exchange rate and probably a fee. Therefore purchase or withdrawal could end up being more expensive.
To avoid a DCC is simple; when you are doing a purchase or withdrawing funds at an ATM and you are offered to be charged in your home currency just reject the offer. You may end up paying more for your purchase or withdrawal. Always choose to be charged in the local currency of the country you are in.