If you write a check for more money then you have available in your checking account then your check will bounce and you'll be hit with an overdraft charge.
And the store or other business you wrote the check too will be contacting you because your check was returned to them and they will be charged a returned check fee which they will make you pay them back the return check fee that they were charged plus the amount the check was written for.
You'll need to pay this back with cash money and if you don't pay back the check and the return check fee then they will likely prosecute you for a bad check and you can end up in jail for writing the bad check.
Other places might use a machine to put your check through right away and if the funds aren't in the checking account then the check will be declined and you'll have to put back what you can't pay for at the store.
Always make sure you have enough funds in your checking account to cover a check you're gonna write.
Otherwise writing too many bad checks can be seen as fraud and can lead to you serving some serious prison time.