Well, assuming I know what I'm talking about, let me give it a shot.
shadow quality
How pixelated shadows will look on the ground/walls/terrain. Higher setting = smoother shadows.
model/texture detail
The maximum resolution of model (players and items) textures and how far away they have to be before the game uses a lower-quality model/texture. Higher setting = guys look great close up and far away, medium = guys look great close up, not so much far away, low = guys look okay close up, not great far away.
shader detail
If I'm right, shaders are things that let textures interact with light, so a rough surface can have shadows to make the roughness appear more realistic. Basically the quality of these.
However, low shader settings can cause other problems (such as the sky disappearing on Peak) and it doesn't seem to have too much of an impact on performance. I'd say keep it on high if you can.
effect detail
I believe this refers to effects that use sprites, such as smoke, muzzle flashes, and explosions. This is the general quality of these. You could probably lower this and not notice too much, explosions and muzzle flashes are quick, and smoke is smoke, it's blurry by nature. You might lose something with fire effects, though.
multicore rendering
Don't really know, sorry. Hopefully someone else does. Or Google, maybe.
anti-aliasing mode
Anti-aliasing basically "smoothes" the edge of objects, especially at a range. I don't remember the options, so I don't exactly know what it means by "mode," though.
filtering mode
Applies a filter to the game, like the green in Fallout 3 or blue to Skyrim, maybe? I'd have to check.
FXAA anti-aliasing
I believe FXAA is full-screen anti-aliasing, and this is generally the standard anti-aliasing option that you would increase for smoother edges on objects. Higher numbers = more anti-aliasing = less performance. 2x is usually the minimum. More is better, but I find 2x to be enough, even for games like Arma 3, pixels really only being noticeable when I'm trying to shoot 200+ meters with iron sights.
vertical sync
Also called vsync, this will lock the framerate at a certain level (usually 60 fps) to eliminate "screen tearing" which is caused by the game displaying frames faster than they can render. Vsync will cause a game with surplus framerate to limit it to a lower level, waiting for each frame to render properly before displaying it.
Most people prefer to have it on, if they can, but some people would rather not take the FPS hit. If your frames are already low, consistently below 24-30, you might want to leave this off, though.
motion blur
Basically, this will blur the screen when you turn the camera. While people argue its realism, in the case of framerate, it only really serves to make the game prettier at the cost of frames. If you're trying to get more frames, this could probably be one of the first things to go.