The Titlebar is at the top of the main AdiIRC window.
By default the Titlebar text is AdiIRC. It can be changed in Options -> Windows -> Main window title or using the /titlebar command.
$titlebar can be used to get the current Titlebar text.
When a window inside the main window is maximized, the window title of the current active channel/server/private window is added to the Titlebar text so it becomes Titlebar text - [Window titlebar text].
The Titlebar text for windows insides the main window can be changed in Options -> Windows -> Channel/Server/Private Titles.
/titlebar can also be used to change the Titlebar text for @custom windows.
The Titlebar can be enabled/disabled by changing Options -> Windows -> Show Titlebar or by changing the Form Border.
When the Titlebar is hidden, you can move the main window by holding the ALT key and the left mouse button while moving the mouse.
The main window icon can be changed in Options -> Icons -> Main.
Changing the main window icon also changes the Taskbar icon and Systray icon.