Alt text its helps part of Image optimization. It's the text that the search engine uses to understand images.
Alt text is the alternative text used by text browsers and other Web user agents that can't view images. It is also one of the only attributes required by the imagetag. By writing effective alt text you insure that your Web pages are accessible, and give you more places to add SEO text that search engines won't penalize you for.
- Log into the Drupal 7 admin dashboard.
- Navigate to the Content types page (Administer > Structure > Content types).
- On the Content types page, find the content type you want to work with and click on the Manage fields link.
- Find the Image field and click Edit link to take you to the settings page.
- On the settings page, scroll down until you see the Enable alt field checkbox and save settings. Check the box to enable alt text for the image.