Select an image in the design to edit SEO Title and Description options. On a gallery, select an individual image in the list of images to edit. Details below on how each setting is used.
Sometimes shown as a tooltip when you hover over the element. Depends on if the element has other interactive functions that would override the tooltip display. Less necessary for SEO.
Applied as ALT text, good for SEO. Used by search engines, screen readers, the browsers used by blind and visually impaired people, to tell them what is on the image.