It's making sure the person absolutely understands what it is you want them to do. For example: Just so I'm clear, the children need to be picked up at 3, not 5. It is normally used if there is some change in plans, or many instructions were given at once and you need to emphasise the most important one. Hope that helps.

I agree with Belinda and Dom. "Just to be clear" is usually used when you are ensuring someone else understands what YOU want. (It can come off as demanding/commanding if you use the wrong tone.) I usually use "to clarify" instead. It doesn't always focus on what I want, but rather to make sure we are both understanding the same thing. 1. Just to be clear, I don't want to you to include any potatoes in my meals this week. 2. To clarify, I don't want to eat any potatoes this week. 3. To clarify, we agreed that no potatoes would be served with any meals this week, right?

"Just so I'm clear" or "Just to be clear" is another way of saying "I want to be certain you understand (the situation), so I'm going to explain some more details on (topic)". When someone is "clear" about something, it means they have made (something) easy to understand because they do not want to be misunderstood. When a person says "Just so I'm clear", it is like saying "Just to be certain that you do not misunderstand me, I'm going to explain this further (or restate what I said before)"