Once you know their names, check them out. It’s useful to check recent publications to get a sense of their own knowledge and research focus. It’s useful to follow that up with a look at their staff pages to see what else you can find out. What are their research interests? What teaching do they do?
It is also really useful to be aware of what they are like as people. Have you met them at conferences? What do you know about your internal? What do their students say?
Knowing their research may give you insight into questions they may have, but knowing about them helps create a picture that these are real people coming to talk to you. Not faceless strangers, unknowable and uncaring: they are humans like you.
Knowing a little about them can help your confidence a lot for the viva.