According to a report from UberGizmo citing Twitter user WalkingCat links to a new 'gamemode.dll' file in the dynamic link library, which hints at provision for Windows 10 to adjust CPU and GPU resources to allocate more resources to the game. The option is aimed to free up more space and resources for the game, which will allow it to run faster and smoother.
With Windows 10, Microsoft has already blurred the line between an Xbox Console and PC gaming device. Microsoft already offers option for cross platform gaming with same titles across PC and Xbox and with game mode, Microsoft is probably realising Windows 10 PCs as consoles by offering more resources for complete gaming experience.
Microsoft is yet to confirm the existence of 'game mode' in any of its Insider build but with Windows 10 Creators Update set to land in early 2017, it is likely to we could see the feature arriving in any of the future beta testing builds.