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