Copied from Halflife2.net.
"The Max frames to render ahead tells the video card how many frames it should render ahead of time, so its at 3 right now, so if your playing a game at 60FPS then its rendering frames that you won't see for 0.01seconds. By always rendering future frames it prevents problems with not rendering the image by the time you see it. The stronger the graphics card you have the further ahead you can set it to render, if you set it to render 100 frames ahead and your running at 100FPS then its currently working on next seconds frames."