Coplanarity

Coplanarity means that things are all on the same flat surface, like a pancake. Think about a piece of paper - all the lines and shapes on it are in the same plane, or on the same flat surface. If you drew a line on that paper that was sticking up off of the page, it wouldn't be coplanar with everything else.

Now let's apply that to 3D shapes. If you have three points in space, they could be anywhere in the air. But if you connect them with straight lines to make a sort of triangle, you've created a flat surface - a plane - that those points are all on. If you added a fourth point, it could either be on that same plane, or it could be sticking up off of it. If it's on the same plane as the other three, they are coplanar.

This can be important in math and engineering when you're working with things like planes, angles, and distances. If you're trying to measure the angle between two lines and they're not coplanar, it won't be the same as if they were on the same plane. Or if you're designing a building, you might need to know if certain points are coplanar in order to make sure everything fits together properly.