If your debit card has insufficient funds then the debit card will not work and you won't be able to purchase anything with it.
When a debit card has insufficient funds it will decline at the register or online and the payment won't go through.
Some banks however do have a insufficient funds coverage though that will cover up to a certain amount should you go over the amount you owe for the merchandise or service.
But if you have no money in your checking account at all then the debit card will decline.