On the screen you'll an arrangement of points (nodes or vertices) connected by lines (arcs or edges). This arrangement is a simple graph, which means that there is no more than one arc between each pair of nodes.
If you choose the "Add arc" option at the bottom of the screen, then dragging with your mouse from one node to another will draw a straight arc between them. If you choose "Delete arc" then dragging from one node to another will delete the arc between them. Choosing "Move node" will allow you drag any node around the screen, together with any arcs that are connected to it.

The "nodes -" and "nodes +" buttons reduce or increase the number of nodes. The "Clear arcs" deletes all the arcs, while the "Complete arcs" draws a complete graph, one where each node is connected to every other one.
You can show or hide the matrix on the right of the screen using the "Show matrix" option at the bottom. You can also show or hide the weights of the arcs using the "Show weights" tick-box.
Play around! Enjoy yourself!