I've never been asked this before, or even ever prepared for it, but it's about showing your eye for detail and meticulousness. Maybe give an example of when you had to proofread something or count something (like money), and go through the process. For e.g., your ability to focus on a task, making sure to be thorough and observant in your reading process, taking extra time to make sure the work is free of mistakes. Also add the impact of your attention to detail, e.g. organisation was kept to a high level, the team was well coordinated etc.
But overall, focus on your actions and how you can demonstrate you are detail oriented.