If I don't understand something, asking someone to clarify on the part I misunderstand, asking someone to repeat what was said (with an apology prior - this is mainly if I completely missed what was said), or to give me insight on the detail(s) I am missing - leaving room for the other person to correct me is also important, as it naturally evokes a response if I have a detail wrong and give the person an opportunity to rephrase it. More importantly, it is active - repeating what someone said is a passive act, as it does not demonstrate understanding, but just that someone knows how to repeat words said, whereas if someone is actively engaged in filling in important details & probing the implications of what was presented, the person is more likely to be focused on explaining the relevant details (& being mentally focused on the topic of conversation) than distracting emotions, thoughts, etc.
If I am confident I understand what is being said, I will operate under the assumption of that understanding, and continue on the natural implications, or on occasion, rephrase what was said to verify if there are major consequences (especially something legally binding).
This approach highly depends on ability to trust (to a degree) though. If one is in a situation where one cannot trust the other person's intentions, then one must be on their guard and rely on their ability to parse a situation & come up with an approach (or delay immediate response before being able to conduct the necessary research in order to make a proper response or have a productive discussion if possible).
To borrow an idea from literature, it is generally better to show than to tell. There is utility to telling, but its utility is mostly in speedily conveying an idea (or ideas) in order to get to more important matters.