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

We know that magnification $m=\frac{size\ of\ the\ image}{size\ of\ the object}$

Here given magnification produced by a plane mirror is $+1$.

So, $m=\frac{size\ of\ the\ image}{size\ of\ the object}=1$

Or size of the image$=$ size of the object

$+ve$ sign indicates that the image formed is virtual and erect.

So, we can conclude that the size of the image is the same as the size of the object and the image formed is virtual and erect.