The alternate text is a text which appears instead of an image on a webpage when the display has been inactivated or when the image file can not be loaded.
The alternate text is also useful when images are blocked by email software clients or webmails.
The alternate text plays a role in SEO because its presence and content are taken into account by most of search engine algorithms. Other things being equal, it contributes to the ranking optimization of the page in SERP. It also may contribute to displaying the image in universal search results and in Google Image SERP.
The alternate text display is achieved by completing the “alt” tag associated to the image tag in HTML code.
In the example below, texts appear in the email thanks to the “alt tag" completion: