What do you mean by the term "Flat Image" and is there any advantage of creating a Flat Image ?

In terms of photoshop, a flat image is an image without layers.

Although layers in photoshop is essential in editing and manipulating your work, alot of layer can be cumbersome and consumes alot of memory.

So it is advised to collapse layers you think you are finished editing, and finally you can collapse the entire set of layer to a flat image to save desk space.