Anyone charged with a criminal offence in Canada is presumed to be innocent. That is just a fancy way of saying that the prosecution must prove someone charged with a crime is guilty. If the prosecution does not prove someone is guilty that case against them fails and they must be acquitted.
What proving someone guilty means is not immediately obvious. It could mean it is more likely than not that they committed the crime. Or it could mean they very likely did commit the crime. Or it could mean there is no reasonable doubt but they committed the crime. In Canada the last of those possibilities is what the prosecution must show.
To be found guilty of a crime, the prosecution must show proof of guilt to beyond a reasonable doubt. This means that the Court must be satisfied beyond a reasonable doubt that the defendant is guilty. The standard of "reasonable doubt" consists of a doubt based on reason and common sense which must be logically based upon the evidence or lack of evidence. It is not based on sympathy or prejudice.
Proof beyond a reasonable doubt does not involve proof to an absolute certainty; it is not proof beyond any doubt nor is it an imaginary or frivolous doubt. The burden of proof much closer to absolute certainty than to a balance of probabilities. The standard is more than proof that the accused is probably guilty in which case the Court must acquit.
Each crime has specific elements that must be proven. The prosecution must prove each of these elements to beyond reasonable doubt. So, in a murder case the prosecution must show, among other things, that someone is dead and that the accused is responsible for the death. It would be an odd murder case where there is any issue over someone being dead but nevertheless that is something that must be proven to beyond reasonable doubt or admitted by the defendant.
An example may help explain what all this means.
Nothing in the life is ever absolutely certain.
Even if ten people saw me hitting my brother it is possible they were all mistaken or perhaps I have an evil twin who really did the beating. Perhaps all ten people were suffering from a delusion and saw nothing. All that could be true but, of course, to believe that would be fanciful. To doubt my guilt would not be reasonable.
A reasonable doubt is a doubt that is reasonable – meaning it is a doubt based on evidence (or a lack of evidence). So, if I were to claim that the ten witnesses who saw me beating up my brother were suffering a delusion and there was no evidence supporting that claim I should be found guilty. But if I showed, say, that the beating supposedly took place at a party and there were bad drugs used by some or all of the ten witnesses I might be acquitted.
Defences must have an air of reality to have any viability. If I want to argue I was too drunk, say, to intend to kill someone (that's a defence to murder) it's not enough to say "maybe I was drunk"; I have to show something the could show I was drinking and therefore was drunk. For there to be reasonable doubt there needs to be something that legitimately gives the Court pause.