Fig. 1. Channel bar window.

The channel bar displays buttons for the Console window, and every other channel/query/dcc chat window you opened. The highlighted button is your frontmost channel window. By clicking a button in this window you can make another window frontmost. You can hide a particular window by holding the Option key while clicking a button. The window linked to that button becomes invisible, but you do not leave that channel.

The buttons show a text balloon icon when there is activity in the related window.

You can resize the buttons also. Move your cursor above the right edge of any button, then click and drag. The button width is stored in your preferences file.

To hide or show the channel bar: hit command-B, or select Channel Bar from the Windows menu.
To move the channel bar: hold the Option key, click in the empty area of the window and drag.