While you can certainly google the settings and get a more detailed answer to your questions, but here's a quick rundown:
Anisotropic filtering makes distant textures look clearer and costs very little performance.
Anti aliasing smiths jagged edges that are caused by low polygon counts, but costs a lot of performance. Worth adding on after you tune the other settings.
Texture quality: Always as high as possible because it has a big impact on looks.
Mesh/terrain quality: Also as high as possible
Shadows: Often a big performance hit at higher levels, so a good one to cut if struggling for performance.
Those are the big ones. Hope that helps.