What does the Response time mean i see some monitors that have a 5ms and some like the dell that have 6ms which response time is better

the DEll 24'' FULL HD looks awsome but what do you guys think of the Acer Computer Australia - (http://www.acer.com.au/acer/akc/monitorslcd.nsf/Page/RWP4FFBAA48B2464CECCA257472004C08A0?open&current=8.7&ms=expand&) for HD Editing