This indicates the length of time a monitor needs to process a request from the computer. The lower the response rate, the sharper the image is during fast scenes. The response rate is especially important for computer gamers. For gaming monitors it should not be more than 4ms.
With a slow or delayed response rate, sometimes things will happen in the game that will be missed completely. That's why the response rate is such an important parameter for players - the lag caused by the low response rate may cause some problematic situations, especially if the stakes are high.