The content contained in images, including pictures, illustrations, and charts, is inaccessible to people who can't see those images. Alt text is added to images to describe the content of the images for screen readers.
What is important to include in alt text?
Alt text should convey the purpose of an image so that if someone can't see the image they can still obtain the important information the image conveys. Alt texts aren't an exact science, but several accessibility sites have suggestions to help you decide what to include in your alt text.
How to verify
The simplest way to verify is to browse your site on staging with a browser that has the WAVE Toolbar installed. Running the "Errors, Features, and Alerts" report will show you where problems exist.
If you cannot install the WAVE Toolbar in your browser, try using the WAVE online web accessibility checker tool instead.