It is said to be a kind of resolution upscaling, and it comes along with something like motion blur. (Of course no one enable motion blur in FPS games) It can produce a slight better graphics, especially when not moving, than scaling down the render resolution.
According to my PC’s (R5 3600X/GTX1070/16GB 3600MHZ DRAM) performance,
in both Temporal Upsampling Ratio and Render Resolution Scale settings at 100%, FPS is around 92
Temporal Upsampling Ratio at 50% and Render Resolution Scale settings at 100%, FPS is around 154
Temporal Upsampling Ratio at 100% and Render Resolution Scale settings at 50%, FPS is around 164
Temporal Upsampling Ratio at 50% and Render Resolution Scale settings at 50%, FPS is around 195
It makes moving objects blurry and like oil paingting.
A document I found about this:
PS: I go for a search about this only because I dont know what it is, and I havent seen this in any games I played before.