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If you are a "basic member" who joined after February 2002, you are limited to 100 posts in a rolling 7-day period. It lets you know when you are getting close to that limit.
A rolling 7-day period means that what you posted last week will drop off your count a week later. If you waited a week and posted 100 times in a single day, you would have to wait seven more days for your post count to reset.