The central theme of a triangle-based modeling system is faceting.
The Unfilled Outline on the left would not render since it is not composed of faces (triangles) and would be considered a hole. The Filled Outline on the right would render as a surface.
There are two ways to facet (make triangular faces so that they fill) an outline ...
| Auto Filling | Using the Fill Outline or Fill to Cursor actions |
| Strutting | Using the Pen Tool |