A bouncer is a software that connects to an IRC Network and stays connected as if it were a normal client.
They usually run on servers and stay online. This ensures your BNC won't disconnect from the IRC server.
The bouncer also acts as a proxy between the IRC network and yourself.
This keeps your IP hidden from the IRC Network, as you connected to the bouncer and the bouncer connects to the IRC Network.
Some of the features of a bouncer include offline channel messages , offline private messages, away nick, automatic identify, and more.