Moral certainty is a term that is used in criminal law to refer to a judge's or jury's strong belief that the defendant is guilty. The trier of fact must have moral certainty to make a conviction.
The term "moral certainty" is often used interchangeably with the term "beyond a reasonable doubt."
The reason why moral certainty, or belief beyond a reasonable doubt, is required for criminal trials is because people who are found guilty of crimes can face severe consequences. So, moral certainty prevents people who may be innocent from being sentenced to prison or facing other consequences.
Of course, this system is not perfect. However, it is currently the method that is used to decide who is voted innocent and who is voted guilty in the United States legal system.