Ok kiddo, so you know how when someone gets sick, we try to keep them away from other people so they don't make them sick too?
Well, a "cordon sanitaire" is basically the same thing, but for whole people groups or areas. If there's a really bad disease that's spreading fast, the government or other people in charge might put a "cordon sanitaire" around the area where the disease is so it doesn't spread to other areas.
That means they'll put up roads, fences, or other barriers to keep people from coming in or out of that area. They might also have people watching and checking to make sure nobody enters or leaves.
It might seem a bit scary, but it's actually a good thing because it helps to stop the disease from spreading and hurting more people. Remember, it's better to be safe than sorry!