Read more and download a Windows build from the BA thread.
To put it simply, “denoising” is a process of analysing your render and trying to shmoosh the noise/grain together and make it look clean.
A common side-effect of all denoising algorithms is that some detail is lost. It’s almost impossible for the computer to tell the difference between noise that you don’t want and noise that you do want (like details in a texture). To address this, as far as I can tell, Lukas has added options to denoise Direct and Indirect passes separately, which allow you to (potentially) smooth only the noise introduced by the rendering process and keep all the details of your textures in tact.
The example above is quite extreme – denoising is meant to be one last final step that will remove just the tiny bit of stubborn and subtle noise and make the image perfectly clean. Why? because the more noise you have to remove, the more details you’ll lose along with it. If you zoom in, you’ll see all sorts of nasty bits and blurred details that will flicker during an animation.
Here’s another great example by LazyVirus: