HEVC Main 10 is a newer profile update to the High Efficiency Video Coding format. It can be found explained in more details at Wikipedia, "The Main 10 profile allows for improved video quality since it can support video with a higher bit depth than what is supported by the Main profile. Additionally, in the Main 10 profile 8-bit video can be coded with a higher bit depth of 10-bits, which allows improved coding efficiency compared to the Main profile."
Three patches to the Radeon UVD code and video acceleration (VA) Gallium3D state tracker implement the AMD bits for HEVC Main 10 decoding. But before getting too excited, this Main 10 profile support is only found for AMD's upcoming Stoney APUs on the open-source driver stack. Current-generation Carrizo APUs with HEVC support only support the main profile.
This work should be merged in plenty of time for the Mesa 11.3 release.