They often provide quick ways to use applications (e.g. iChat) or display information (for example the system clock), or control system-level variables (for example audio volume).
Menu extras are similar to items in the Microsoft Windows notification area but are less common.
There are many menu extras supplied with macOS, many independent third-party menu extra applications, and many more supplied with 3rd-party products, most of which are installed from their parent application or system preferences pane, and may remain dormant until they are notified of an event (for example, Inkwell's menu extra will come and go when a graphics tablet is connected and disconnected).
While macOS provided no centralized tool to enable or configure menu extras, some of them can be rearranged and dragged off the menubar while depressing the ⌘ key.
There was some controversy when they were originally introduced due to Apple providing an API for third-party developers to use to create menu extras using the NSStatusItem class, while Apple used another private class called NSMenuExtra to develop their own menu extras.