Frame Rate is the refresh rate - 30fps generally pleases most though some people find that choppy. To get higher fps you need a video card/computer that can support this and a monitor/tv than can support it. Most tvs and monitors can support 60fps but a lot of them can’t support higher than that. FPS can be impacted if you are trying to show too much detail - the video card may not be able to render higher detail at your targeted FPS.
FPS is the smoothness,
Graphics quality is typically represented as two things 1) The number of pixels or resolution (eg 1920 by 1080) and 2) the details (shadows, anti-alias, other options). It appears that Zwift really only gives you the option of specifying the resolution. Most of us will opt for the highest resolution that our system can output - which is limited by the TV or monitor and graphics card we have as well.
You can improve FPS by upgrades to video card but it may be constrained by the TV or monitor you have connected to it (refresh rate of your screens may be lower than what your video card can produce)
You can improve the resolution by upgrades to video card (it can handle rendering the zwift world in 4K potentially) but again your monitor can constrain this.
So you want realism - you want 4K output and at a sufficiently high FPS so it doesn’t bug your eyes. So you need a more power video card and a monitor that can handle it. No easy answer as to what to buy as there are so many options. Check out some video game blog sites for information.