A D-Link Microfilter (or Splitter) is used on a phone line or extension where both an ADSL / VDSL modem as well as a PSTN (i.e. analogue telephone, fax machine etc) device is desired. The unit splits the line, providing a pass-through connection with no filtering to the ADSL modem, while providing a filtered connection to the PSTN device.

Note that all extensions other than your ADSL / VDSL Modem connection need to have a filter of some kind placed on them.