Reasonable doubt refers to a standard of proof that judges or jurors are supposed to use as a basis for convicting accused criminals.
If the prosecution has proved that the defendant has committed the crime "beyond a reasonable doubt," then judges or jurors are supposed to vote guilty. However, if a reasonable doubt remains, then they are supposed to vote not guilty, according to the legal system.
The reason why the legal system uses this standard of proof for conviction is because if a reasonable doubt remains, then it is still not clear whether or not the person committed the crime.
Criminal sentences can be very lengthy and difficult, and so convicting an innocent person can be a truly undesirable circumstance. However, there is no objective means of determining whether or not guilt has been proven beyond a reasonable doubt. Cases vary dramatically, and it is up to each judge and jury to decide this for himself or herself.