It's called a shebang. It's the *nix way of determining what interpreter to use when executing a shell script. The operating system looks for the shebang at the start of the script its executing, and then passes the entire contents of the file to the program specified by the shebang. Any additional arguments are appended to the end of the program.
For instance, say I have a file called hello.fb:
#! /usr/bin/foobar -y
All of this text as well as the line above
is passed to "foobar -y" as STDIN
Then this line:
Is the same as:
/usr/bin/foobar -y world < hello.fb