Flatten image is basically the quick way of merging all the layers together using what you see on the screen as what comes out at the end.  This avoids the problem sometimes of the image changing slightly depending on what order you merge the layers together.

Hope that helps (and as far as i know it is correct )

"In every colour, there's the light.
In every stone sleeps a crystal.
Remember the Shaman, when he used to say:
Man is the dream of the Dolphin"

Rich Meadows Photography