Dynamic res will lower the render resolution depending on a target FPS, there needs to be more varables added for it to work (if it actually even works on PC, more used on console)
Basically it will try to maintain a target FPS and if it dips below that point for X amount of frames it will lower the render resolution by Y % (that you can specify min/max resolution) to maintain performance.
Its a good trick but it can lead to a muddy/less crisp image when it does it, and if used too drastically it will make the game look like 720p or less :P (or like a console)
Audio qquality level is just as it says, but counterintuitivly 0 is actually the highest, i dont think it will do anything as i imagine the assets/sound files have to be made in a lower quality for that to work.
HDR is just if u have a HDR monitor, self explanatory