Alt text stands for alternative text and is used by web browsers to describe images. By writing effective alt text you have more places to add SEO text that search engines can crawl.
The longer your alternative text, the more difficult it will be to read by text browsers. It can be tempting to write long sentences of alternative text, but keeping them short keeps your pages smaller and smaller pages download faster.
A good rule of thumb for alternative text is to keep it between 5 and 15 words.
Good for SEO
People often forget that alternative text can be a great place to put search engine keywords. But remember that your first priority is to your readers. Keyword spamming in alternative text can be detected and search engines change their rules all the time to prevent spammers.
A good rule of thumb is to use your search engine keywords where they fit with the description of the image, and don't use more than one or two keywords in your alternative text.