Using the system header file stdbool.h allows you to use bool as a Boolean data type. true evaluates to 1 and false evaluates to 0.

#include <stdio.h>
#include <stdbool.h>

int main(void) {
    bool x = true;  /* equivalent to bool x = 1; */
    bool y = false; /* equivalent to bool y = 0; */
    if (x)  /* Functionally equivalent to if (x != 0) or if (x != false) */
    {
        puts("This will print!");
    }
    if (!y) /* Functionally equivalent to if (y == 0) or if (y == false) */
    {
        puts("This will also print!");
    }
}

bool is just a nice spelling for the data type _Bool. It has special rules when numbers or pointers are converted to it.