The filter cap is the part that holds the filter against the bottom of the AeroPress chamber. The filter cap is designed to be used with AeroPress paper micro filters (sold separately). Popular with coffee enthusiasts worldwide, the patented AeroPress is a new kind of coffee press that uses a rapid, total immersion brewing process to make smooth, delicious, full flavored coffee without bitterness and with low acidity. Good-bye French Press! The rapid brewing AeroPress avoids the bitterness and high acidity created by the long steep time required by the French press. Plus, the AeroPress paper Microfilter eliminates grit and means clean up takes seconds.