Addons is what we call the dexi app store. Here you'll find some addons that'll add new actions to your Extractors and Pipes - and also more integrations to 3rd party applications like Google Sheets. 

To activate an Addon simply choose the one you want and click "Add" in the sidepanel:
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Note that most integrations allow you to add as many of them as you need - so if you have several FTP servers or Google Sheets you simply add another instance of the Addon for each of them.