could you explain me what its exactly will do?
$who and $logfile = a script variable, defined by the writer. Would have to see more of the script to tell you.
>> = append the output of a command (or in your case, value of the $who variable) to the end of a file.
2>&1 means to direct both STDOUT and STDERR to the same place, typically for logging purposes or even to suppress output of a script (like with cron)
In conclusion, it will append STDOUT and STDERR of $who to $logfile