When using a DSL connection with a telephone or fax machine, it's necessary to put a filter on the line to separate the signals for the modem (higher-frequency ADSL) from the signals for the phone and fax (lower-frequency POTS).
This filter/splitter has three RJ11 jacks to do just that.
Connect the 'line' jack to your wall outlet cable, and connect the 'phone' and 'modem' jacks to your phone or fax line, and to the modem.
Each jack is conveniently labeled. Signal quality is preserved and your connections stay clear of static and noise from interference.