The function printf is used to print output to the screen.  An example is

printf("hello world\n");

In between the double quotes is the character string which is printed on the screen.
Special character strings include the \n in the double quotes which corresponds to a newline character.
This advances the line when printing on the screen.

Variables can also be output to the screen.  A program segment is shown below.

#include <stdio.h>

main(void)
{
int days;

days = 7;

printf("The number of days in one week is %d\n", days);

return 0;
}

the variable named days is output on the screen using the integer character, %d.
The output of this program would be:

The number of days in one week is 7

See ABC for other typical special character strings.