This refers to how much bigger (or smaller) the image is compared to the object.

Magnification = m

                         image height =  v
                         object height       u
where
v=distance of image from optical centre and
u=distance of object formoptical centre

Example

An object 4cm high, 8 cm from a lens forms an image 2cm high. How far is the image from the lens?

                         image height =  v
                         object height       u

       2         =   v
       4             8

v = 4 cm