If the game supports Windowed mode (not Fullscreen) you can alt tab out and interact with the 2nd display or whatever else, while still having the game visible. Otherwise in Fullscreen mode your mouse is usually locked to the game and you'll only be able to see what is on the second screen (this is usually the best purpose for a 2nd monitor, to keep tabs on something else). Alt tabbing will minimize the game if you want to actually do something on the 2nd monitor.