IRC (Instant Relay Chat) is a protocol that allows a large number of people to create or join a chat room to carry out discussions. Many people in the free software community use it to connect with each other and collaborate on projects. Some call it old-fashioned, we call it "timeless."
Connecting to an IRC network requires a dedicated program, called an IRC client. On GNU/Linux, XChat, HexChat, and Pidgin are among the most popular. To use IRC through the terminal, the most popular options are irssi and WeeChat.
Once you install any one of these, enter the server and channel names to connect.
One of the most popular servers for free software is freenode. Some free software channels you might want to join are: #fsf, #gnu, #dbd, and #libreplanet. Most free software projects have their own dedicated IRC channel, such as #bash and #emacs.