How much is expected to know before starting a PhD
The best way to respond to this specific question is to engage the potential supervisor about specific content in the articles.  You are not expected to know everything at this stage.  But it is important to show you are interested, can engage with the material, and (not least) actually took the time to read what they sent.  

A response like "I read them but I didn't understand everything" doesn't give them any information about you.

Be specific, e.g.


I found the way authors applied X method in this paper to be really interesting.  I was familiar with this but didn't realize it could be used this way.
The authors did X but why didn't they use alternative approach Y?
I don't understand what this statement means...could you explain it?


Asking questions about things you don't understand can be one of the best ways to engage.