A widget is a “window object”. It is something that is displayed that has at least two parts: a state and a behavior. An example of a widget is a button. Its state is things like what color is it, what text is written it in, and how big it is. Its behavior is things like what it does when you click on it, or what happens when the cursor is moved over or away from it.
In Tcl/Tk there are three parts to creating a useful widget. The first is creating an instance of the widget with its initial state. The second is giving it a behavior by defining how the widget behaves when certain events happen — event handling. The third is actually displaying the widget possibly in a group of widgets or inside another widget — geometry management. In fact, after creating all the widgets for a GUI, they are not displayed until handled by a geometry manager, which has rules about how to calculate the size of the widgets and how they will appear in relation to each other.