Drag the mass and release it to start the oscillation. Grab it again in mid-swing and it stops where you hold it, then starts over from wherever you let go. The "To rest" button returns both the displacement and the velocity to zero.
In the chain panel you add and remove springs and masses to link them in series, and set the spring constant, coils, mass, and damping by number. The equation and readout panels show the period, the energy, and the displacement and velocity of the selected mass. The panel names in the top right open and close each panel, and panels can be dragged around.