"Alt text (alternative text) describes the appearance or function of an image on a page. Alt text is read aloud by screen readers used by visually impaired users, displays in place of an image if it fails to load and is indexed by search engine bots to better understand the content of your page." [from - What is alt text?]
From Butler University LibGuide
Describing complex images such as graphs, charts, diagrams, and maps requires a two-part alternative text.
Learn more at W3C Web Accessibility Initiative Complex Images tutorial.