This is a version of Send Rich Email that includes a Custom Property Editor. It requires a Spring '20 org that has been enabled for the Custom Property Editor pilot. To instead look at the release version of Send Rich Email that works on unmodified Winter '20 orgs, see https://github.com/alexed1/LightningFlowComponents/tree/master/flow_action_components/SendHTMLEmail
Learn More: https://unofficialsf.com/send-rich-email-with-the-new-sendhtmlemail-action/