Further information about sampling can be found here.
More information about working with sampling and removing noise can be found here.
Sampling is divided into the following sections:
Locks the AA_seed so that the sampling noise won't change with the frame number (avoiding the film grain look).
If enabled, Arnold takes into account the effect that autobump has when computing the raytraced BSSRDF's result.
This helps capture the high frequency details of the surface more accurately, at the expense of slightly longer render times.