What do these codes do?

I saw this source code in a book:

#include <stdio.h>
#include <unistd.h>


int main(int argc, char *argv[])
{
    char *delivery = "";
    int thick = 0;
    int count = 0;
    char ch;

    while ((ch = getopt(argc, argv, "d: t")) != EOF)
        switch(ch)
        {
            case 'd':
                delivery = optarg;
                break;

            case 't':
                thick = 1;
                break;

            default:
                fprintf(stderr, "Unknown option: '%s'\n", optarg);
                return 1;
        }

        argc -= optind;
        argv += optind;

        if (thick)
            puts("Thick Crust.");

        if (delivery[0])
            printf("To be deliverd %s\n", delivery);

        puts("Ingredients: ");

        for (count = 0; count < argc; count++)
            puts(argv[count]);

    return 0;
}

I can understand the entire source except:

argc -= optind;
argv += optind;

I know what is argc and argv, but what happen them in these two lines, what is "optind" Explain me.

Thank you.