Fidelity Mode: This will focus on the game's resolution, producing full 4K graphics while scaling the frame rate down to 30fps to focus on the visual output.
Performance Mode: This mode focuses a lot more on the frame rate. It will bring the game up to 60fps while not trying as hard to keep the resolution at a constant 4K picture. With upscaling, it may hit 4K but will most likely fluctuate from 1440p upwards (unless you are on the Xbox Series S, which will decrease to 1080p).
Performance Mode RT: This is akin to a mixture of the two modes above. It will work hard to hit a constant 60fps while also providing a 4K picture. Due to it focusing on both elements, however, graphics and frame rate wise won't be a fixed rate and will dip to lower frame rates or resolutions to handle the game when it gets intensive, such as when the screen is densely populated.