An overdraft is a negative balance that the bank allows on your account: you can spend more than you have, up to an agreed limit, and pay interest on the borrowed amount.
In plain words
It is a short-term credit line attached to a current account or card. While you have your own money you spend it; when it runs out, the bank pays the difference from the credit limit. The debt must be repaid under the contract, and interest and fees depend on the bank: some offer an interest-free period, others charge simply for using it. So check the rate, the limit and the repayment rules before you use it.
Example. Your card holds UAH 1,000 and a purchase costs UAH 1,500. If the card has an overdraft with a UAH 5,000 limit, the payment goes through and the account shows minus UAH 500, which you must repay under the contract (the numbers are illustrative).