A nanosecond (ns) is one billionth of a second (10-9 or 1/1,000,000,000 s)...( From Wikipedia http://en.wikipedia.org/wiki/Nanosecond )

As an example, a variable with data type TIME may store a data like 08:31:50.0000005 which means the time now is 08 Hours, 31 Minutes, 50 Seconds and 500 NanoSeconds

Similarly, I hope you can understand the meaning of other accuracy constraints for TIME/DATE data types.