Understanding PRIntervalTime

Hello,

I do not understand the meaning of the following sentences from

http://www.mozilla.org/projects/nspr/reference/html/prinrvl.html#20947

"Interval times are accurate regardless of host processing requirements
and are very cheap to acquire.

The constants PR_INTERVAL_MIN and PR_INTERVAL_MAX define a range in
ticks per second. These constants bound both the period and the
resolution of a PRIntervalTime object."

(?)

Greetings,
Joachim