In a nutshell, game runs at lower resolution, but DLSS fills the rest of the screen with AI generated pixels to improve the quality. It's cheaper performance wise to do this than render actual high res objects.
#WHAT İS NVDIA DLSS
urban escarp
lusty jungle
It's talking about VRAM since rendering the game at a lower internal resolution means you save on some VRAM/memory. DLSS is Deep Learning Super Sampling, just a form of Anti-Aliasing & Upscaler with performance benefits. DLSS is not worth using in RDR2 though cause of the artifacts, you're better off without it at least in this game.