A higher pixel resonse time (measured in ms) means that pixels will change state faster. At 60Hz, anything over 16ms technically means no ghosting should occur, but since the listed number is only an average, this typically wasn't the case.
As a general rule, for 60Hz, 5ms and below is fine. If the moniter goes higher though, you'll want a lower Pixel Response Time to minimize ghosting. Just remember the listed number is a best case scenereo.