Most of the answer people put are incorrect.
"Negotiation of meaning" is an educational term.
It means that when someone is speaking, you ask them questions to elaborate in more detail what they are talking about.
EXAMPLE:
Teacher: How did you enjoy your vacation to Germany?
Student: It was fun.
Teacher: What was fun?
Student: It was fun to see stuff there?
Teacher: What exactly?
Student: I enjoyed seeing the buildings.
Teacher: Which buildings did you see and why did you enjoy them?
Student: I enjoyed the architecture of the Cologne Cathedral.
etc.
So you are having the student speak, and guiding him or her with clarifying questions to help them speak more and in more detail.
That is what I gathered from a Google search in:
"Negotiation of meaning theory"