You've probably already seen one of the following lines:
#!/bin/sh
#!/usr/bin/env python
#!/usr/bin/env python3
#!/usr/bin/env python
#!/usr/bin/perl
#!/usr/bin/php
#!/usr/bin/ruby
This is a shebang. It's a directive for your command line interpreter how it should execute a script.
For example, you have a file with this content:
#!/usr/bin/env python3
# -*- coding: utf-8 -*-
import sys
print(sys.version_info)
Now you can execute it via python3 yourFile.py. But alternatively, you can make it executable and simply type ./yourFile.py
By the way, you should use #!/usr/bin/env python for some reasons.