longdesc attribute provides a description field for images that are more information-rich than can be conveyed in an alt attribute, such as in graphs or infographics. This uses the WordPress media library’s “Description” field to generate a page linked to from the
longdesc attribute which can describe that image.
The description field supports HTML, so you can add structure to the content in this field to help users understand the content.
WP Accessibility has a couple of options for how the information will be presented: either as a link that follows the image or as a button that displays the information as an overlay on top of the image.