An irc bouncer or BNC is a piece of software that is used to relay an irc connection. Using a BNC allows a user to hide the original source of the user's connection, providing privacy as well as the ability to route traffic through a specific location. A BNC can also be used to hide the true target to which a user connects.
Another advantage is that many BNCs remain connected to an IRC server in the event the client disconnects from the Internet. Often state changes are tracked so that they may be relayed to the client upon reconnection.