c Questions about language structure

#include <stdio.h>

struct test
{
    int a;
};

int main()
{
    struct test num;

    num.a = 3;

    printf("%d\n", num.a);
    printf("%d\n", num);

    return 0;
}

The output from printf("%d\n", num.a); in the above code is 3. And printf("%d\n", num); below it is also printed as 3. Here's my question. How on earth does num have 3? For me, num.I put the value in a. I understand that structure test serves as a data type, but num plays a role other than connecting the values of the structure?