More commonly known as a modem port, phone connector, phone jack or phone line, the Registered Jack-11 (RJ-11) is a four or six wire connection for telephone and Modem connectors in the US. The picture is an example image of what the RJ-11 phone cable and its connection.
Do I need to connect the phone cord?
If your computer has a modem, it likely included a phone cord. Unless you plan on connecting to the Internet with a dial-up connection, you do not need to connect the phone cord, and it can be discarded. Today, most people are connecting to the Internet with a broadband connection and most new computers no longer have a dial-up Modem.