The Focal Plane is the area in which the subject of your image is in sharp focus. The focal plane spans horizontally across your frame, all the way from the left to the right. Picture it as a line of tape across your shot. No matter how many subjects you have in your shot, as long as they are standing along that line, they will be in focus. If they move in front of or behind that line, then they will suddenly be out of focus because they have stepped out of the focal plane.