It's a Frames Per Second setting. The normal game caps FPS at 30, but you can inactivate it so it runs off of what your computer is capable of. It can make the game look better, but your computer perform slower.
The main thing I noticed was with arrow keys. On PC, you can use arrow keys/WASD to move around. With the 30FPS cap on, you move around at a normal pace. But with it off, you move a lot faster. Other than that, I couldn't find any differences.