Please Discribe This Program For Me

[rpapaiof]

 #include<stdio.h> 
#include<math.h>
#define mod %
int main()
{
    unsigned char flag,counter=0,rows=0;
    long unsigned int number,i,j;
    printf("\n\t Enter your number:");
    scanf("%du",&number);
    for(i=1;i<=(long unsigned int)abs(number);i++)
    { 
        flag=0;
        j=2;
        while(j<=(long unsigned int)sqrt(i))
        {
            if(i mod j==0)
            {
                flag=1;
                break;
            }
            j++;
        }
        if(flag==0)
        {
            printf("%10lu",i);
            counter++;
            if(counter==10)
            {
                printf("\n");
                counter=0;
                rows++;
            }
            if(rows==20)
            {
                rows=0;
                printf("\n\t\t Press any key to view next page... \n");
            }
        }
    }
    printf("\n");
    return 0;
}