Linear Magnification = height of image / height of object = - v / u
This is the result of the cartesian sign convention that is adopted for mirrors.
If this is negative, it means that the image is inverted with respect to the object. You see the image upside down on the screen. If object is above principal axis, then image is below the axis and vice versa.
If it is positive , it means that the image is upright or erect with respect to the object. It could be on the screen (real) or inside the mirror (virtual image). The image and object are on the same side of principal axis.