What does "1>&2" mean?

Shell Programming and Scripting
1 
if [ $# -ne 3 ]; then
            echo 1>&2 Usage: $0 19 Oct 91
            exit 127
fi

What does "1>&2" ? Please let me know

2

1 - stdout
2 - stderr

This means redirect echo's output to stderr, with a message on how to use the script,
there needs to be exactly three arguments.

All error conditions should go to stderr.