what are these numbers in c language ?

#include <stdio.h>

int main(){
    FILE *file = fopen("test.txt", "ab");
    
    fwrite("hello world", 1,4, file);
}

#include <stdio.h>

int main(){
    FILE *file = fopen("test.txt", "ab");
    
    fwrite("hello world", 1,4, file);
}

what is

and

??

if i am not mistaken

is the number of bytes to read and

is the size of the byte which means i can increase the size of

to wherever i want to present unicodes am i wrong ?