求解这个代码的意思，及要实现的功能

string GetRandomString()
        {
                const int SIZE_CHAR = 8; //生成32 + 1位C Style字符串
                const char CCH[] = "0123456789abcdefghijklmnopqrstuvwxyz";
                srand((unsigned)time(NULL));
                char ch[SIZE_CHAR + 1] = {0};
                for (int i = 0; i < SIZE_CHAR; ++i)
                {
                        int x = rand() / (RAND_MAX / (sizeof(CCH) - 1));
                        ch[i] = CCH[x];
                }
                return ch;
        }