Some higher-end MacBook Pro models come with two separate Graphics Processing Units (GPUs). The first is integrated into the CPU. This is used day-to-day. The second is a dedicated “discrete” graphics GPU made by either Nvidia or AMD, and is usually called upon for photo/video editing tasks and 3D games.
Why not just use the discrete graphics all the time? Because it burns through battery life. A MacBook Pro with a nine hour battery is typically reduced to just five hours when using discrete graphics. Sometimes even less.
Many apps like to switch the Mac to discrete graphics, including the Photos app built into OS X, and Adobe Photoshop. Aside from a slight flicker on the screen, it’s impossible to tell when this is happening and the first you’ll know is when the battery starts to drain more quickly – by which point it’s too late.
gfxCardStatus is a free menu bar app that will tell you via a notification message when your Mac has been switched to discrete graphics by an app. Its menu bar icon will also change. It offers a menu option to force the Mac to use only integrated graphics, or to force it to use discrete graphics all the time.
In short, it’s a must-have app if you’ve a MacBook Pro with discrete graphics.