It is screen reaction time, ie the time it takes for your screen to react to any command. It refers to how quickly a pixel can change colours, measured in milliseconds (ms); the fewer the milliseconds, the faster the pixels can change, reducing the ghosting or streaking effect you might see in a moving or changing image. If you are an avid gamer and movie buff and intend to watch movies or play games on your PC, then you need to get a monitor with a good response time. The average RMS (response per millisecond) is 8. The really high-end monitors come with 2 RMS.