Game Mode is, reportedly, a feature that will allow Windows 10 to adjust both CPU and GPU resources in order to allocate more resources for a game.
This way, all the other applications will be given a secondary priority and your games should have access to more GPU and CPU power (and, perhaps more RAM as well).
This new Game Mode should be usable in both old and new games (Win32 and UWP games).
What this means for our simming? If all runs according to plan, this will mean that our simulators will have more resources allocated as well. Although some of the sims out there may not be affected in a big way, we should see some performance improvement for those that use Windows 10 as their simming operating system.
Looks like Microsoft's taking gaming serious.