In the viva it means supporting your thesis.
It means restating what you did, or replying to a question about a related aspect. It could be listening to another point of view and reconciling it with your work. It could be clearing up a mistaken belief that your examiner holds. It could be making your work clearer because it wasn’t clear in your thesis. It could be all of these things and other things besides.
Defending your work does not mean being defensive. Defensive is not listening. Defensive is thinking that you are right, no matter what. Defensive won’t help.
You could be angry or upset with a comment or question, you can feel what you feel – but you’ll serve your thesis and your viva better by defending your work, rather than by being defensive.
Listen to the question or comment. Check it against what you think and feel. Think about your response. Check it for emotion – you don’t need to be a robot, but be careful you’re not just reacting to a negative feeling.
Defend, not defensive.