What if I do not understand the answer?

Say which part you followed and ask them to continue from there: I follow the first half, could you take it from the weighting? That is more useful than I still don't understand, because it tells the professor exactly where the explanation broke, and it usually produces a second, shorter explanation rather than a repetition.

Name the last point you did follow and ask them to continue from there. That gives the professor a place to restart, which usually produces a shorter second explanation instead of a repeat of the first.

How to Talk to Professors in English: Asking a Question in a Large Lecture →