What is the resistance, initial resistance, the final resistance?

Resistance: fiber to bypass the airflow, resulting in small resistance. Countless fiber resistance and the resistance of the filter.
Initial resistance: The air resistance of the tested filter that does not accumulate dust through the rated air volume.
Final resistance: refers to the rated air volume, due to filter dust, and the initial resistance rose to the specified value, usually twice the initial resistance can also be recommended by the manufacturer, the unit with Pa