If a device is running at 30 frames per second, and the screen has a refresh rate of 60 Hz, then 30 images per second will be shown to the viewer. Even though the screen can display more images per second, it is only getting 30 images per second from the device. However, the device runs at 120 frames per second, then only 60 images per second will be shown on the screen. This is because the screen can not display more than 60 images per second (its refresh rate).