The full form of MMCSS is Multimedia Class Scheduler Service

Multimedia Class Scheduler Service (MMCSS) is a Windows service that allows multimedia applications to get prioritized access to CPU for time-sensitive processing (such as multimedia applications) as well as prioritized disc access to ensure that the process is not starved of data to process. The MMCSS service monitors the CPU load and dynamically adjusts priority so that the application can use as much CPU time as possible without denying CPU to lower priority applications. MMCSS uses heuristics to determine the relative priority required for the task the thread is performing and dynamically adjusts priority based on that. A thread must invoke MMCSS explicitly to use its services by calling the AvSetMmMaxThreadCharacteristics() or AvSetMmThreadCharacteristics() APIs.

MMCSS is used by the multimedia applications in Windows Vista, including Windows Media Player and Windows Media Center to provide glitch-free audio playback.