Response time is simply, the time taken by a monitor to go from complete white to complete black or from complete black to complete white.
In case of a LCD monitor, it is the speed by which the monitor’s pixels could change from one colour to another. It is measured in milliseconds.
A computer monitor with a faster response time will be good for playing fast paced games.
In the earlier days, CRT monitors had a lead over the LCD monitors for higher response times. But with the current technology LCD monitors also provide higher response times making them ideal for gaming purposes.