The viva is an oral exam at the end of the PhD. You submit a written thesis based on your research in advance. Typically two examiners, one internal and one external, will read your thesis carefully. In the viva they facilitate a discussion with you.
The viva is different for every candidate but there are common expectations. Most candidates pass. Most candidates are asked to make amendments to their thesis.
Nervousness is common, but only a symptom of how important the viva is. Candidates can prepare and rightly feel confident of their success given their experience, work, talent and knowledge.
I am sometimes asked very simple questions about what the viva is and what happens. I make assumptions sometimes about what someone might know, and get puzzled looks about certain details. The three paragraphs above are my attempt to share “the basics” in 100 words. What do you think?