Defrag options in Windows Server 2012

Retrim and SlabConsolidate are for thinly provisioned storage volumes.

-ReTrim Generates TRIM and Unmap hints for all currently unused sectors of the volume, notifying the underlying storage that the sectors are no longer needed and can be purged. This can recover unused capacity on thinly provisioned drives.

-SlabConsolidate Indicates that the cmdlet performs slab consolidation on the storage to optimize slab allocations and to reduce the number of used slabs.

I don't know for certain but my guess is the -$ changes the IO priority to run as a back ground process.

Powershell guide for optimising volumes. http://technet.microsoft.com/en-us/library/hh848675.aspx