It's a way of defining of which shell should be invoked during execution of the script. It's called the Shebang.
Example, a file having the following Shebang line:
would invoke the Bourne (sh) shell. So for example, your piece of script:
## Setting a default route
/sbin/route add SUBNET $5
/sbin/route add SUBNET $5 run in the Bourne shell interpreter.
/sbin/route add SUBNET $5 is just a comment and doesn't do anything. However, the shebang should be on the first line, therefore that comment at the top will disallow the shebang to do its work, making it a useless addition.
There's a number of interpreters that are widely used and some typical interpreters for shebang lines are:
#!/bin/sh — Execute using sh, the Bourne shell (or a compatible shell)
#!/bin/csh — Execute using csh, the C shell (or a compatible shell)
#!/usr/bin/perl — Execute using Perl
#!/usr/bin/python — Execute using Python
#!/usr/bin/php — Execute using PHP's command line interpreter
As Olli mentioned in his comments, sometimes
env (short for environment) is used to make sure the correct path to the interpreter is chosen.