what is the definition of open expression?

Answer 1

You didn't give any specific context, but in general it seems to mean "freedom of speech, demonstration or collective expression" e.g. in a university or public setting.
http://www.vpul.upenn.edu/osl/openexp.html

Alternatively if you mean "open statement/question" it would mean a statement/question which does not presuppose an outcome, e.g. "Do you understand what I'm talking about?" or "I'm not sure you understand what I'm talking about"
instead of
"You don't understand what I'm talking about, do you?"