As above it will render at a higher resolution and then scale it down, it can then look smoother. At the cost of performance. This is however, sometimes slightly better than running anti aliasing. (sometimes)
It's kind of pointless to run both simultaneously as you're then super sampling a super sampled image so your vRAM will be being used pointlessly. Turn Anti Aliasing off, and turn super sampling up as far as you can before your frame rate tanks

Keep an eye on your VRAM usage and when you hit the sweet spot, stick to that.
the number btw is a multiplier, so 1.5 super sampling is your resolution, bumped up 1.5 x and then sampled down. Example: if you're running 1920x1080 and you run 4x you'll then be rendering at 4K and sampling it down to 1080p to fit into your screen. (and it looks lush)
The issue with nVidia's DSR is that you then need to change your mouse tracking speed, I find you don't need to do this if you use in-built versions of super sampling.