Difference between pass by reference and pass reference by value


I understand that in ruby, like java, when a method takes an argument and modifies it, it has the reference to that object, meaning that it modifies the contents of that object. So without returning anything, the original object passed as an argument is changed.

This is what I understand by "pass-by-reference", but I have read that Ruby is "pass-by-value" or "pass-reference-by-value". I do not understand what the difference is here.