If you write a check to someone or a business when you have insufficient funds to cover that check then the check will bounce.
The check will be returned to the business and you'll have to pay a returned check fee which can be as much as $50.00 and you'll have an overdraft fee of $30.00 or whatever the bank charges.
Basically your check will not be honored for the purchases or payment when you write a check with insufficient funds in your bank account.
Stores will usually run your check right through a machine and if the funds are not in the checking account then the purchase won't go through.
Writing checks with insufficient funds can be costly and could cause the bank to close your bank account and make it harder to open another bank account and it could land you in jail if you're considered to be writing the bad checks on purpose.