What does a negative magnification of a mirror or lens actually mean?

The sign in magnification tells you the orientation of the image, not its size. Magnification m is the ratio of image height to object height, and it is also equal to -v/u for mirrors or v/u for lenses depending on convention. A negative magnification means the image is inverted (upside down) compared to the object, which always corresponds to a real image. A positive magnification means the image is upright (erect) and virtual. The number part tells you the size: |m| greater than 1 means the image is bigger (magnified) and |m| less than 1 means it is smaller (diminished). So m = -2 means the image is real, inverted, and twice the size of the object. Always read the sign and the number separately.