A GPU is essentially a second CPU for your computer. The difference is that while a CPU is good at any task you can throw at it, a GPU is really good at exactly one thing – performing the kind of complex mathematical calculations that are used to render graphics. A CPU could technically perform these operations, but it would just be a lot slower at it.

When you are playing a game that has fancy HD graphics and needs to run at a high FPS, the CPU can offload the rendering to the GPU and the GPU sends the final frames to the display directly, resulting in much faster performance.