Image description field

Any time you upload an asset to your website (image, PDF, video, or audio file) WordPress, by default, creates a page to host just that piece of content.

That page gets its own URL which can contain more information about the asset than may be necessary or relevant on its parent page.

So the description is the descriptive text that shows up on your image attachment page.

You can read more about the fields here: https://limelightdept.com/articles/the-difference-between-image-alt-texts-and-descriptions/

Thanks for the brief, excellent answer!
jmt