The requirement of "causation" means that a person who is being tried for homicide must be shown to have caused the victim of the crime to die. This sounds like a pretty obvious thing, but it is not always so simple.
In many cases, the causation is obvious. The defendant shot the victim with a gun and the victim died. Clearly, the defendant caused the death of the victim.
But causation is not always so obvious. Let's say that someone drives drunk and crashes into someone else's car, killing them. This could be a homicide. But what if the person who died was not wearing a seat belt? Which thing killed them? Was it the crash? Or was it the fact that they were not wearing the seat belt because, if they had worn the seat belt, they would have survived?
There are no clear ways to define causation -- to define exactly when someone has caused the death of another. But causation must be found in order to prove a homicide.