The same factors that make a photograph grainy apply here.

Film ISO (Speed) - lower for less grain (Kodachrome at ASA 25 gave remarkable detail in it's day)
F/Stop - besides governing amount of light, depth of field (relative amount of blur) decreases with higher values.
Shutter speed may needs to be changed for any adjustment in above values.

You also may have to let it run for more iterations.  If your max samples size is too low, the render will complete before max quality is reached.
I use 5000 to make sure quality doesn't get cut off.

Your monitor pitch can also be a factor, but most modern monitors are around 0.28mm which is fine for most purposes.

I tend to set it for 50000 iterations. (Use the little Cog to adjust the parameter min and max or just turn off the limits there. :)