The threshold period is the length of time that status messages are counted before the counts are reset to zero. If there were no threshold period, every status message threshold, no matter how high, would eventually be crossed, but it would not be meaningful; a hundred messages in three months is not as significant as a hundred messages in one day.
Therefore, choose a threshold period appropriate for how often you plan to check the status summarizer. The threshold period should be the same as or greater than how often you plan to check the status summarizer; that way, any threshold crossings will not be reset before you have a chance to look at them.
If you want to reset the message count on any particular component or site system, you can right-click it and choose Reset Counts.