an image representing a web page, intended to accompany a link when the link is shared
An image which is memorable and relevant to your website or page, and which is displayed when you share a link on social media.
When a link is shared on FaceBook or Twitter, the platform scans the "header" of the HTML of the web page, and if the Open Graph "meta tags" are defined, loads a brief description and an image to diplay with the link.
Use of a hero image is important to SEO because it helps the algorighms in our minds understand and index the link to your site in our memories.
It's important to understand the feedback loop between the search engine algorithms and the algorithms in our minds.