What is option "Square Sampling" and AA ?

What is option “Square Sampling” and AA ?

It takes your typed in sample count and multiplies it by itself, thus “squared”. The AA stands for Anti-Alias. So if you square the default setting of 10 when you issue a render Cycles will actually use 100 samples instead of 10.

Activate the checkbox and issue a render, observe the sample count along the top of the screen as it renders.

Yeah I discovered that it is the Make Render Slowly button.

I added it. It helps tweak values for sampling and brings Cycles in line with sampling in Arnold, MR, and others. The longer an image renders, the more samples it takes to clear remaining noise. Squaring samples ensures that you’re A.) Working will small, easy to remember numbers and B.) Increasing sample rate at an amount that will have a noticeable impact on render quality. This way you can have settings like 4AA for preview, 6AA for final, 8-10AA for motion blurred/DOF shots, etc.