toolkit.checkImage - ImageObserver??

a pretty simple question…

What does the ImageObserver parameter do in the method toolkit.checkImage(image,width,height,imageObserver); ?

The method states it does not begin loading of an image, so what is the ImageObserver needed for?

Can images have only a single ImageObserver to monitor their loading?
and if so, is that what this parameter does?
(re-assign the images ImageObserver to the 1 specified)

If thats not what it does, what on earth does it do!