What is the 'Use AMD FSR 1.0' option on the pre-game Options window?
AMD FidelityFX Super Resolution 1.0 is one of two upscaling techniques used in Total War: WARHAMMER III. Upscaling is used whenever the in-game Resolution Scale option is set below 100%, i.e. when the game's 3D graphics are rendered at a lower resolution than the 2D interface.
By default, the games uses Contrast Adaptive Sharpening for upscaling. Enabling FidelityFX Super Resolution 1.0 will improve the quality of upscaled 3D graphics at the cost of performance, as it is more demanding.
If the Resolution Scale is set at or above 100%, neither upscaling technique is used. You can adjust the Resolution Scale by selecting Options > Graphics > Advanced > Resolution Scale from the Main Menu.