Pass by reference passes a pointer to the value. This allows the caller to modify the variable directly.Pass by value gives a copy of the value to the caller. This allows the callee to modify the value without modifying the variable. (In other words, the callee simply can not modify the variable, since it lacks a reference to it.)
First read the answer fully, then try to explain it in your own words. After that, open a few related questions and compare the concepts. This method helps you remember the topic for a longer time and improves exam preparation.