Question

The magnification produced by a plane mirror is +1. What does this mean?

Answer 1

Magnification, $m=\frac{Imageheight}{Objectheight}=\frac{h_i}{h_0}=\frac{v}{u}$

The magnification produced by a plane mirror is +1 which means that size of the image formed is exactly equal to the size of the object behind the mirror.
The positive sign shows that the image formed is virtual and erect.