After pressing the New button, clicking gives you new nodes for the graph.
After pressing the Sink button, clicking on a node makes that node a sink.
After pressing the Join button, clicking on a node and then on another gives you an edge of the graph.
After pressing the ++ button, clicking on a node increments its number of chips.
The SingleStep and Run buttons execute the process.
For explanations, see the theory page. For the sandpile version (chip firing on a large grid), see another applet.