The Heading section contains the Title field, the name of the widget as it appears in the widget area under the Home tab in the left sidebar. You are allowed a maximum of 255 characters (including spaces), and the field is mandatory.
The Content area beneath contains controls to define what the widget shows to members. Its nature depends on what the widget displays, and how you define that content (this example screenshot is from the HTML Note widget).
For the relevant details for each widget, see: Widgets reference