What is it?

Supersampling is an anti-aliasing technique, which attempts to eliminate pixelated or jagged edges (see shadows in non-supersampled image below).  Aliasing occurs because real world objects have continuos smooth curves which cannot be properly modelled in a pixel grid (discrete values).

The Basics

Supersampling is easily achieved by rendering the image at a much higher resolution and then downsampling it to the desired size using the extra pixels for calculating the average colour. You can also view it as that you are taking samples within each pixel. The method of sampling can very the results of the anti-aliasing achieved. In order to offset some of artifacts caused by certain sample techniques, I opted for a stratified sampling or jittering technique.