Hi Colin
These are the parts of the workpiece from which references are made. I was taught, "Take the best and broad side, plane it flat and smooth". This then becomes the face side, and is marked with and old-fashioned curly f for Face.
Then the best-looking edge is planed straight and square to this Face. That becomes the Face Side, and is also marked, I use a V. The Face Mark and the Edge Mark are usually drawn adjacent to each other. With these two straight, square and true surfaces, all the others (like thickness and width) can be gauged, cut and planed. Without them, it's difficult to ensure that a piece of wood is anything like the drawing says it should be!
I hope this is a start.