The Linux Command Line (책) - Redirecting Standard Error : 이 부분이 이해가 되지 않습니다.

To redirect standard error we must refer to its file descriptor. A program can produce output on any of several numbered file streams. While we have referred to the first three of these file streams as standard input, output and error, the shell references them internally as file descriptors 0, 1 and 2, respectively. The shell provides a notation for redirecting files using the file descriptor number. Since standard error is the same as file descriptor number 2, we can redirect standard error with this notation:

[me&linuxbox ~]$ ls -l /bin/usr 2> ls-error.txt
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http://linuxcommand.org/tlcl.php 여기 보시면 PDF를 확인하실 수 있어요.

위에 서술한 부분이 이해가 안돼요. Redirection Operator는 뭔지 대충 알겠는데, File Descriptor가 뭔지를 잘 모르겠네요.. 처음 보는 단어라서.. 개념이 잘 안잡하는 거 같아요.
설명해주실 분 계신가요?