The most common form is alt text on images, but text alternatives also cover things like a text transcript for an audio clip, or a visible or programmatically associated label for an icon-only button. WCAG’s guidance (success criterion 1.1.1) is that every non-text element needs an equivalent purpose-appropriate alternative, except purely decorative images, which should be marked so screen readers skip them entirely.
Good alt text describes the image’s function or content concisely, in context, rather than restating obvious visual details or stuffing in keywords.