Remember: In math, a plane is a flat, two-dimensional surface that stretches infinitely far in all directions.
An example of a plane would be the photo below.
The purple rectangle is one plane, and the brown one is another one. If there are points there, then they would be on both planes if the points lie exactly on the intersection between the two planes.
We label planes when they have a letter (K, M, etc.), or if there are three non - collinear points on one plane, we take those letters and make them into the plane's name. (ex: Plane AGE, QDE, etc.)