Program to understand how a structure variable is returned from a function

#include<stdio.h>
struct student
{
char name[20];
int rollno;
int marks;
};
void display(struct student);
struct student change(struct student stu);
main( )
{
struct student stu1 = {"John", 12 , 87 };
struct student stu2 = {"Mary", 18, 90};
stu1 = change(stu1);
stu2 = change(stu2);
display(stu1);
display(stu2);
}
struct student change(struct student stu)
{
stu.marks = stu.marks + 5;
stu.rollno = stu.rollno - 10;
return stu;
}
void display(struct student stu)
{
printf("Name   - %s\t", stu.name);
printf("Rollno - %d\t", stu.rollno);
printf("Marks  - %d\n", stu.marks);
}