Note that for the save feature to work you need write permission on the directory from which you are running. The other gadgets let you change the boundary conditions. The program starts with open boundaries and sand is lost for any tumbling on the edge. Periodic boundaries connect the left and right sides as well as the top and bottom. In this case sand is never lost and it may be that avalanches never end.
Sandy boundaries represent an infinite source of sand just outside the lattice.
Every step a grain is tossed on each edge site and two on each corner site. Try the flow boundary condition to see what it does. With the mouse, scribble some sand randomly on the system. Wait a few minutes until the system stabilizes and activity ceases. Now you should be in the critical ensemble.