"If two points lie in a plane, then the line joining them lies in that plane." For that postulate in Geometry, there is something I don't get. A plane is a shape with edges that go on forever, right? So it means if there is a plane, then covers the entire flat surface. So even if on the picture, the points are on the "outside" of the shape representing the plane, it still means they are in the plane right? Sorry if this is confusing to some people, I tried to make it as simple as possible but I guess I kind of failed, haha.

Anyways I would appreciate any answer explaining this and why. Thank you!