The button object is one of the most basic UI objects in Max. When clicked on, it sends out the "bang" message. If you send it anything via a patchcord, it will briefly light up, and the input will trigger an outgoing "bang" message. You can use button objects as triggers in your patcher. They can also provide useful graphical feedback about your patcher's activity and message passing.
The keyboard shortcut for creating a button object in a patcher is the letter "b"
Here is a patcher which demonstrates some of the characteristics of the button object, and UI drawing in Max in general
(please list things that you believe to be errors or omissions from the existing refpage)