UK finance jargon - in plain English
Spending more than you have in your bank account.
An overdraft lets you spend more money than is currently in your current account, up to an agreed limit. Your bank effectively lends you the shortfall. There are two kinds: arranged (agreed in advance) and unarranged (unplanned, often with much higher charges).
Your balance is £50 but your rent direct debit for £600 goes out. With an arranged overdraft, the payment clears but you now owe the bank £550 and interest starts from that day. It feels invisible, which is what makes it dangerous.
Numbers simplified for illustration. Actual rates and tax rules may change.
People think overdrafts are free money. Even arranged overdrafts can charge high interest rates, depending on the bank. Treat it like any other loan and clear it as quickly as you can.