Answer:
To put it simply, its the bodily, I guess, movement that harms another that can be considered a crime. Like, pulling a knife on someone, or something. Anything from physical assault to murder, to petty crimes in those areas, would all be classed as an actus reus.
As for mens rea, it is not only the action but also the evil mind. So, the intent behind a crime. Mens rea translates to "guilty mind".
These elements are important because they help determine whether or not a crime has been commited and whether or not it was intentional.