The 'ms' stands for milliseconds, it is a unit of time. In the context of PC monitors, it refers to the response time of the monitor. Monitors with high response times can highlight ghosting or blurring of the image, but this is generally something of the past as the response rate of monitors have improved significantly. This means you should buy the monitor with the lowest response time available, often 2, 5ms as they are the best for gaming any higher and It's no good really!