For example #ls 2>&1 > ls.log
What is actually mean by 2>&1??
Since every shell process has there own descriptor ::
1 (stdout) and
2 (stderr) :: and & in the front of &1 indicated that 1 is not a file or folder, it is a descriptor. The > mean redirect the out put to another descriptors or files. in this case 2>&1 > ls.log means that stderr(2) and stdout(1) put is redirected to the ls.log file.
normally, for example #ls > ls.log without 2>&1, means stdout(1) is redirect to ls.log and stderr(2) will show in the console.
I hope this will help you to understand more about the descriptor redirection.