The prosecution has the burden to prove the defendant is guilty. The defendant is not required to prove he/she is not guilty. Juries must presume that the defendant is not guilty until such jurors are convinced from the evidence that the defendant is guilty. If a juror has a reasonable doubt as to the truth of any of the claims required to be proved by the prosecution, such juror must find the defendant not guilty. If a juror has no reasonable doubt as to the truth of any of the claims required to be proved by the prosecution, such juror should find the defendant guilty.
One may observe that the term "innocent" does not appear in the foregoing discussion of stages of a criminal case. That is because a prosecutor generally litigates only whether a given criminal defendant is "guilty" or "not guilty" - as measured by the applicable burden of proof.
The term "innocent" connotes a complete lack of involvement and/or culpability. A defendant may be shown to have played some role in a crime, in which case he or she would not be purely "innocent". However, if the prosecution cannot prove that the same defendant is guilty by proof beyond a reasonable doubt as to each element of the charge, then the defendant is considered "not guilty."
"Beyond a reasonable doubt" is a high burden, and rightfully so, because the penalties in the criminal law include deprivation of liberty (going to prison). Conversely, the burden of proof in a civil case (over money damages) is "more likely than not", which is a lower standard than that in a criminal case.
The burden of proof in a criminal case is "beyond a reasonable doubt." It is not "beyond a shadow of a doubt," "beyond any doubt," or some other extreme high burden such as "absolute certainty." "Beyond a reasonable doubt" means that there may indeed be some doubt in a juror's mind, but the question becomes whether that doubt is reasonable. The prosecutor may not be able to prove that space aliens, for example, did not materialize and break into the bank vault - but is that a reasonable possibility? Of course not.
One might believe that a juror must be convinced of guilt to a 100% certainty in order to vote to convict. However, the only way for such a level of proof to be achieved would be if the juror had actually witnessed the crime. Witnesses testify at a trial, they do not sit as jurors.
The burden of "beyond a reasonable doubt" is not insurmountable. It is the same burden of proof that has been the law in the United States, and it is the same burden that applied in all criminal cases for which convictions have been obtained since the beginning of our country's history.