When you don't understand try saying:
- "Could you say more so I can get a better idea what you mean?"
or
- Do you mean X or Y?" to then work out a yes / no and get more details from there.

Try above instead of just saying "I don't understand".

This way, even if X and Y are completely wrong, the other person explaining will have a better sense of where you are and can adjust their explanation.

A kind of triangulation as long as someone isn't offended that you assumed they were talking about it differently and said X or Y but checking / saying your guess does help see if you know what they are talking about.