What is the filter?

<span style="font-weight: bold;">Airgle Air Purifier:</span><br /> <ol> <li>Turn off and unplug the unit </li> <li>Press the light button to the "on" position </li> <li>Plug in the unit, and then release the button after the Filter-check starts</li> <li>The system will automatically reset the Filter-Check Gauge in about 25 seconds</li> <li>Once the procedure has completed resetting, please turn on the unit and press "Filter Check" button again.</li> <li>This time, the system should report an accurate result. </li></ol><span style="font-weight: bold;">Alive Air Purifier:</span><br /> <ol> <li>Insert a straightened out paper clip into the reset hole, with the unit off, and push the Timer button.</li> <li>You should hear a 2 beeps, signaling that the Reset was successful.</li></ol><span style="font-weight: bold;">Holmes Air Purifier:</span><br /> <ol> <li>Units which have an indicator light must be reset once the HEPA filter has been replaced. Reset indicator by inserting a small object (such as a pin or paperclip) into the hole marked reset.</li></ol><span style="font-weight: bold;">Honeywell Air Purifier:</span><br /> <ol> <li>Reset the filter indicator by pressing and holding the "Filter Monitor" button until the indicator light shuts off.</li></ol>or<br /> <ol> <li>With the unit powered on, press and hold the appropriate filter<br />indicator button down for approximately 10 seconds until the<br />light turns off.</li></ol><span style="font-weight: bold;">Hunter Air Purifier:</span><br /> <ol> <li>Change the filter and/or pre-filter.</li> <li>Now, you must choose which counter you want to reset. Press and hold the filter reset key (approximately three seconds) until the prefilter indicator flashes. Press and release the reset key again to alternate between the two indicators. The counter that will be reset is designated by the flashing indicator. For Models 36394 and 36395: Use the tip of a pen or pencil to press the key.</li> <li>Once you have the correct indicator flashing, press and hold the reset key for three seconds to reset that counter to zero hours. The indicator will stop flashing when it has been reset. If needed, repeat these steps to reset the other counter.</li></ol><span style="font-weight: bold;">IQAir Air Purifier:</span><br /> <ol> <li>To reach the Filter Life Reset function from the main display window, press the Menu key seven times.</li> <li>Press and hold the Enter key until the flashing cursor appears. Release the Enter key. The first filter selection to appear is the Pre-Filter.</li> <li>Press the Up Arrow key to select the filter that has been replaced. The Filter Life LED for the appropriate filter is now flashing red.</li> <li>Press the Enter key to confirm that the selected filter has been replaced. The filter life LED for the appropriate filter is now flashing red.</li> <li>Press the Enter key one more time to confirm the filter change or to exit without resetting the filter life, press the Menu key (M=Menu).</li> <li>Upon successful reset of the filter life, the new remaining filter life (taking the currently selected fan speed into account) will be displayed, and the filter LED light on the control panel will change from red to green.</li></ol><span style="font-weight: bold;">Kenmore Air Purifier:</span><br /> <ol> <li>Press the FUNCTION and OFF TIMER buttons at the same time for five seconds. The CHANGE FILTER INDICATOR will flash five times and turn off </li></ol>