In practice, this means images need text alternatives so screen reader users know what they convey, video needs captions for deaf and hard-of-hearing users, and content can’t rely on color alone to convey meaning, since color-blind users may not perceive the distinction.
It also covers things like ensuring text can be resized up to 200% without losing content or functionality, and that audio doesn’t auto-play in a way that drowns out a screen reader. Perceivable failures are often the most visible and easiest to catch in a basic audit.