Alt text should communicate the image’s useful meaning, not every visual detail. Decorative images can use empty alt text so assistive tech skips them.
ELI5
Think of Alt Text as a named building block in a big LEGO app. Text that describes an image for screen readers and fallback contexts.
In practice
Use it when you are mapping how the app is structured or explaining a feature to a teammate or agent. In practice, define the owner, input, output, and failure mode before you rely on it.