This simulation deals with longitudinal standing waves that can be set up in a pipe that is open at both ends, or closed at one end, such as the pipes in a pipe organ. In the simulation, for the open pipe (the pipe that is open at both ends) you can select from the fundamental frequency of the pipe, or the second, third, fourth, or fifth harmonic. For the closed pipe (closed at one end), you can see the fundamental, or the four lowest harmonics. You only get the odd harmonics of the closed pipe, where a harmonic is defined as a frequency that is an integer multiple of the frequency of the fundamental.
Standing waves are produced from constructive interference between two identical waves, one traveling to the right and one traveling to the left. Completely constructive interference occurs only in particular cases, when there is a special relationship between the wavelength of the traveling waves and the length of the pipe (the length happens to be 2 m, in this simulation). You can examine that special relationship with this simulation. For reference, the transverse representation of the wave is shown at the top of the screen.