In C++, "call by value" is a parameter passing mechanism where a copy of the actual argument's value is passed to the function's formal parameter. This means any modifications made to the formal parameter inside the function do not affect the original argument's value outside the function. It ensures data encapsulation and prevents unintended changes to the original data. However, it may involve additional memory usage due to copying large objects. Call by value is the default parameter passing method in C++, providing a secure and isolated environment for function execution without altering the caller's data.