Alt text is the alt attribute on an image. Its first job is accessibility: a screen reader reads it aloud for people who cannot see the image, and it is shown if the image fails to load. It also gives search engines and AI tools a text description of what the image contains.
Write it as you would describe the image to someone over the phone: specific, brief and relevant to the page. "Dashboard showing monthly signups rising after the launch" is more useful than "graph" or a string of keywords. Do not start with "image of", and leave the attribute empty for purely decorative images so assistive technology skips them.
Text inside an image is not reliably read by crawlers, so put important information in real page text too. This supports AI-Readable content and general technical SEO hygiene.
Related: Search Engine Optimization, H1.