Acceptance criteria are the specific conditions a user story must satisfy to be accepted, written so they can be checked or tested.
They make a story's scope clear before work starts and give testers something concrete to verify. A common format is Given, When, Then.
Example
Story: "As a shopper I want to save my cart." Criteria: Given I am signed in, when I add an item and leave, then the item is still in my cart when I return; the cart keeps items for 30 days.
In Starlex
Write acceptance criteria in the issue description or as a checklist, so each one can be ticked off. See issue tracking.