The '@' symbol means the variable is an attribute (attribute, @ttribute, get it?
) of whatever class it's declared in. Attributes live in an instance of the class (an object), and is alive as long as the object is alive. Without the @ symbol, the variable will be "dead" once the method it is declared in finishes, which is not what you necessarily want. In this example, it doesn't matter so much. Since GTK will maintain a pointer to the drawing area as long as the window is open, it isn't really needed, but if you ever wanted to expand, and refer to that variable in other instance methods of the class DAWindow, you'll be able to.
Edit: If you want to learn more about Ruby, you should check out Why's Poignant Guide. It's fun to read, and teaches you quite a bit about ruby as a language.