This box contains the names of common channels, so that you don't have to search for them. You don't have to keep the default channels in this box, you can remove them and add new channels at any time. You can also prevent the box from appearing by un-checking the box at the bottom of the window. There are many IRC channels. A large network, such as EFNet can have hundreds of channels available at any given time.
Users can join existing channels or create new ones.
If you were looking for a channel about dogs. This command would bring up a window with all the channels that contain the word 'dog' in their topic. This new window is the channel. Once you're in a channel, type what you want to say and press enter when you're ready to show it in the channel. You should see what you just typed appear with your nickname next to it.