It’s called a shebang, and tells the parent shell which interpreter should be used to execute the script.
#!/bin/sh <--------- bourne shell compatible script
#!/usr/bin/perl <-- perl script
#!/usr/bin/php <--- php script
#!/bin/false <------ do-nothing script, because false returns immediately anyways.
Most scripting languages tend to interpret a line starting with #
as comment and will ignore the following !/usr/bin/whatever
portion, which might otherwise cause a syntax error in the interpreted language.