In any criminal case. The prosecution has to prove their case against a defendant “beyond a reasonable doubt.” This is the highest standard we have in our legal system. Although the term is used commonly, it is hard to define.
One common misconception is that proof “beyond a reasonable doubt” means proof beyond all doubt. This is not true. There is some doubt in almost every criminal case because the possibilities of something happening or not happening are endless, especially when factoring in the human imagination. The standard is for "reasonable doubt," not all possible doubt. Beyond a reasonable doubt does not mean a strong probability that the defendant is guilty.
Something is proved beyond a reasonable doubt when a jury (or judge in some circumstances) determines that the charge is true to a moral certainty - meaning the highest degree of certainty possible in matters relating to human affairs.
The prosecution has the burden of proving a person is guilty of a charge beyond a reasonable doubt. This burden does not shift to the defendant to prove they are innocent. This is because a person is presumed innocent in our criminal justice system.
If after a trial, the defense can show there is reasonable doubt that the charge is true to a moral certainty, then the person is entitled to be found not guilty and acquitted of the charges.