If you spend more than you have in your bank account, you are said to be in ‘overdraft'.
If you tell your bank in advance that you will be overdrawn they will usually agree an overdraft limit with you. This is an authorised overdraft and some banks may charge you for this service and for interest on the borrowing.
If you have not told your bank in advance that you will be going overdrawn then your overdraft is unauthorised and it is very likely that they will charge you, sometimes quite heavily, on top of interest for the borrowing as well.