Significato "%" ?

#include <stdio.h>
#include <math.h>

int main (void)
{

    double amount;
    double principal = 1000.0;
    double rate = 0.05;
    int year;

    printf("%4s%21s\n", "Year" , "Amount on deposit");

    for( year =1; year <= 10; year++) {
        amount = principal * pow(1.0+rate, year);
        printf("%4d%21.2f\n", year, amount);
    }

    return 0;

}

Non ho capito questo pezzo, il libro non lo spiega bene

for( year =1; year <= 10; year++) {
amount = principal * pow(1.0+rate, year);
printf("%4d%21.2f\n", year, amount);
}

soprattutto la parte :

"%4d%21.2f\n"