Alt text, short for alternative text, is a descriptive attribute used in HTML to explain the content of an image on a webpage. It appears when you hover your mouse over an image or when an image fails to load, providing context about what the image represents. Alt text is also crucial in accessing the web, as it allows visually impaired users to understand images through screen readers. Additionally, it plays an important role in SEO by helping search engines index and understand the image content, which can improve a website’s search ranking. Well-written alt text should be concise, relevant, and informative.