Criminal act, criminal intent and concurrence, to use the non-latin, legal terms. i.e you have to prove a crime was committed, you have to prove that there was intent to commit the crime (though this one is fairly weak, you don't have to know you are committing a crime) and you have to prove that the person had the opportunity, or concurrence.
In other words, was there really a murder, did the defendent intend to commit the murder and do the two elements concur, i.e. was the person there at the time, and did they commit the murder.
Beyond reasonable doubt is the level to which you must prove the 3 elements.