At the viva you’re doing more to tell your examiners about what you’ve done, how you did it and who you are. You’ve set that out in your thesis but now they need more from you.
They need you to do more to clarify what you did.
They need you to do more to explain what it means.
They might need you to do more to convince them of an opinion you hold.
They can ask you to do more to tell the story of why your research makes a contribution.
Whatever they need you to do, however, it’s more of what you’ve already done. Doing more is showing your knowledge, your ability, your thinking and all-round capability as a researcher.
It’s only a little more though. That’s all you need and all your examiners need for this final challenge.