The filter is a movement threshold.
It is a distance from 0 (start position) to the setting value of filter, e.g. 30 cm, within which the microcomputer does not display any measurement. If you perform an exercise with a range of motion shorter than filter value (30 cm) e.g. 25 cm, the microcomputer does not show you the result. But if the displacement is longer than filter value (filter 30cm), e.g. 35cm, the microcomputer displays this repetition.
The reason of the filter is to filter out unwanted movements, e.g. taking the barbell from the rack to the starting position etc.