https://compositing.wordpress.com/2010/09/13/premultiply-unpremultiply/
A premultiplied images is an image that has the RGB multiplied by the alpha channel.
This is usually for creating transparency when merging with a background (with the over operation). However problems occur when colour correcting the premultiplied image.
‘Unpremultiple’ is the process of dividing the alpha out of the image. Then the colour corrections can be done. After this stage, the alpha can then again be multiplied back in. This can often be done within the colour correction node itself, or manually.