what does this do?

void some_function(char*** arr, const char* s, size_t *arr_len) {
    if (*arr == NULL) {

        *arr = malloc(sizeof(char*));
        (*arr)[0] = strdup(s);
        *arr_len = 1;
        return;
    }

    char** tmp;
    if ( (tmp = realloc( *arr, (*arr_len + 1) * sizeof(char*) )) == NULL) {
        perror("couldnt get more memory.");
    }

    *arr = tmp;
    (*arr)[(*arr_len)++] = strdup(s);
}