Acceptance criteria are clear conditions that a piece of work must meet before people consider it finished. They are like a checklist agreed on before ordering a custom item.
For a sign-in page, a criterion might say that a person sees a clear message when the password is incorrect. This makes the expected result easier to understand than a vague request such as “make sign-in work well.”
Good criteria describe what someone can observe or test. They help a developer, an AI coding assistant, and the person requesting the work aim for the same result.