No. It defines the virtual plane the the information is flipped over.
Think of putting a plane where you want to mirror it over. In your case, it would be in the YZ, as there is no X depth to it. If you rotate this plane around, say so it's lying on the X axis, the plane you would want to mirror over would be the XY, as there is no Z depth to it.
Understand? It's a bit hard to get your head around. If you know the theory behind 3d planes from beginners calculus, you will understand a bit more.
Imagination is more important than knowledge.