In digital marketing, Conversion refers to the point where the recipient of a marketing campaign performs the desired action in response to the campaign.
For Email campaigns, Conversion can be measured by the response received to your Call-to-Action, for example, the clicks received on the action buttons or images, etc.
Email Conversion Goals help you set Conversion Goals for your Email campaigns.
Suggested Reading: How to set Email conversion goals? How to monitor Email conversion goals?