What are double pointers in C?

main.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void display(char********* output) {
    printf("%s\n", *output);
}

int main(int argc, char* argv[]) {
    char* str = "This is a test";
    char** str2 = &str;
    char*** str3 = &str2;
    char**** str4 = &str3;
    char***** str5 = &str4;
    char****** str6 = &str5;
    char******* str7 = &str6;
    char******** str8 = &str7;
    char********* str9 = &str8;
    
    display(str9);
    return 0;
}