Progressive rendering is a rendering technique in which the program gradually updates small parts of the entire image refining it from low quality to the final result rather than focusing on one small part of the image at a time. The point of progressive rendering is to always see the process of refining the rendered image as a whole, pretty much like a painter sees his work evolve from sketch by looking at the whole canvas after adding more and more detail.
In Corona, progressive rendering goes on infinitely by default. You can however stop it manually, or use rendering limits ( 3ds Max | C4D ) to make it stop automatically.