This is a simple spring system: a number of weights are free to move along a single axis; each is pulled towards its nearest neighbours with a force proportional to the distance between them. A driving force is applied, the speed and power of which can be varied. Clicking in the display area pulls the nearest weight towards the cursor. When you release the cursor the loop twangs back.
The coupling constant - the strength of the attraction between neighbouring weights - controls the speed of the wave. The length of the wave is proportional to the speed it travels at, and inversely proportional to its frequency.
To make wave packets, put the driving force right down to zero. Then move it smoothly to the right and back again.