What are the pleas and what do they mean?

“Guilty” simply means you are admitting guilt to the charge;
“No Contest” means that while you admit some of the facts could lead toward your guilt, you would like a chance to explain the circumstances; and
“Not Guilty” means that you feel you were not guilty of the charge and you would like a trial so the Magistrate can hear from all parties involved and make a decision based on the testimony.