Dynamic Refresh Rate Switching is a power-saving feature that allows for the graphics driver to switch between low and high refresh rates based upon the system's usage scenario. If the system can get by with a lower refresh rate in an idle or low-usage desktop setting over say gaming where a higher refresh rate is preferred, the system will dynamically move down to the lower refresh-rate.
Dynamic Refresh Rate Switching is dependent upon having an internal embedded DisplayPort (eDP) panel and support for DRRS must be advertised by the panel's EDID data.
In the patch-set by Intel's Vandana Kannan, the Intel DRM driver can now support idleness detection within the display driver and switch to the lower advertised refresh rate. More details can be found via the patch-set on the intel-gfx mailing list that could make it into the Linux 3.14 kernel.