If you have insufficient funds in your bank account and try to withdraw those funds either through your debit card or by going to the bank the transaction will be declined or the bank teller will tell you that you do not have the funds available in your bank account to withdraw those funds.
You can only withdraw the money from your bank account if you have sufficient funds in your bank account to cover that amount of money you're withdrawing.
You could write out a check with insufficient funds but the check will be returned to the person you wrote it out too and you'll be charged a return check fee as well as an overdraft fee which is pretty expensive.
So you cannot really buy anything without having money in your checking account.
Your debit card would also be declined at the store or wherever you tried to use it for purchases if you have insufficient funds.