PythonScriptHandler function is used to trigger python script in your execution.
Firstly you have to write your python script under Data > Manage python scripts.
Note you have to use the name (as highlighted in the above snapshot)of the python script in your python script handler function.
Select SET command from the action dropdown and provide a variable name in Element key where output will be stored, edit parameter section and click on Form, select functions from the dropdown and select PYTHONSCRIPTHANDLER function. Provide name of the function which you have given in Data > Manage python scripts.
The output of the script will be stored in a variable (output) defined in element key section. Once you complete the step it will look as below: