This is the estimated amount of time a particular air purifier filter will last.
Filter life is difficult to estimate since the speed of the fan and how dirty the air is both effect how long a filter effectively lasts.
Some air purifiers have a built in timer that measures the average fan speed and how long the fan has been running to provide you an idea of when to change the filter.
A dirty filter will be ineffective at cleaning the air (and may actually release pollutants into the air).
Filter cost can be expensive so the longer a filter is built to last, the better.