In plain language
Every meaningful image, icon, chart, or other non-text object needs a text alternative that serves the same purpose, while purely decorative images must be hidden from assistive technology.
Who it affects
Blind and low-vision people using screen readers, and anyone who browses with images disabled, when icons or photos carry meaning.
How to check
Turn on a screen reader (NVDA/VoiceOver) or inspect the code: every meaningful image should announce a sensible description; decorative ones should be skipped silently. Worked example — A subscription box service's account settings: compare the alt text word-for-word to the current image content: if it describes something else, it's wrong and fails regardless of length or effort.
Common misconception
Adding some alt text isn't the goal. Wrong or lazy alt text (“image123”, keyword-stuffed SEO text) fails just as badly as no alt text, and marking a real photo as decorative hides real content. Real example: on a recipe site's ingredient list, an image that carries real information is added through CSS as a background image, so there is no alt text anywhere — screen reader users never know it exists.