This node is designed to modify the input geometry. If you just want to run a script whenever the network cooks, see the Script SOP.
This node lets you modify geometry in a network using a quick ad-hoc script. To create a new, reusable geometry node type using python, see creating a geometry SOP with Python.
See editing geometry using python for information on how to write the script.
Python code should modify the incoming geometry.
When enabled, the underlying Python interpreter will not be cleared between cooks. This may give some performance gains, but can also lead to unexpected behaviour. It is generally recommended to leave this disabled.