I can explain it:

  m = n
    ? /* if (n != 0) */
      /* adds index.html if path ends with "/" (means the filename is omitted), otherwise copies zero */
      strcpy(b+i-1,b[i-2]-'/'?"":"index.html"),
      /* log the requested filename to stdout */
      printf("%s\n",b+5),
      /* if "/." is in the path or an error occurred while opening the file */
      strstr(b,"/.")||!(f=fopen(b+5,"rb"))
        ? "404 Not Found"
        : "200 OK"
    : "501 Not Implemented"; /* if (n == 0) */

By filtering filenames with "/." I prevent exploits with ".." and also don't allow to read files starting with a dot, these are hidden files in Unix-like OS.