Lower response times are better. It is how quickly a pixel on an LCD screen can change.
Generally TN monitors with very low response times will have poorer color reproduction but are good for games and fast-paced movies. (Less ghosting)
Monitors with slower response times (higher) will be IPS/PLS or IZGO monitors. These will produce accurate colors and are found in the graphics industry, though these days many are good enough enough for gaming at 5ms.