Use pass by value when when you are only "using" the parameter for some computation, not changing it for the client program. In pass by reference (also called pass by address), a copy of the address of the actual parameter is stored.

Is it better to pass-by-reference or value?

Pass-by-references is more efficient than pass-by-value, because it does not copy the arguments. The formal parameter is an alias for the argument. When the called function read or write the formal parameter, it is actually read or write the argument itself.

What does it mean to pass by value?

"Passing by value" means that you pass the actual value of the variable into the function. So, in your example, it would pass the value 9. "Passing by reference" means that you pass the variable itself into the function (not just the value). So, in your example, it would pass an integer object with the value of 9.