ReShade is a post-processing injector program, built with the express purpose of pumping up otherwise saggy graphics. While there are a handful of programs that can be used to accomplish this, the most frequently-used one is called ReShade.
This includes things like cleaner shadows and lighting, less glitchy surface textures, realistically swaying grass, and all that other fun stuff you’d see in an “RTX On” meme. “Shaders,” in the case of Roblox, is a catch-all term for any kind of external software or mod intended to increase the visual fidelity of the game.