Think of looking through a peephole or a concave lens at a tall object; the object appears smaller than it actually is. The 'magnification' in this case tells you what fraction of the original size the image has become.

In optics, the linear magnification ($m$) of a lens is given by the formula $m = \frac{h_i}{h_o}$, where $h_i$ is the height of the image and $h_o$ is the height of the object. For a concave lens, the image formed is always virtual, erect, and diminished, meaning the image height is always less than the object height, resulting in a magnification value between 0 and 1.

Always check sign conventions in lens problems; for virtual images formed by concave lenses, $h_i$ and $h_o$ are both measured upwards from the principal axis, keeping magnification positive.