That's sound. Very lossy sound, presumably at a very low frequency, but sound nonetheless. There's a physical medium, and a signal is being passed through it via tennis ball pressure. If you had a sufficiently dense field of tennis balls, you could visually observe the wave moving across it as the field compressed (in the direction the signal is traveling) and then uncompressed, cyclically.

A good trick to telling the difference is thinking about the direction of the frequency. Frequency is a back and forth movement. If it's going back and forth in the direction the overall signal is moving (like a tennis ball going faster towards the destination and then more slowly or backwards, or like the wall at the source of the signal vibrating towards the other wall), that's a physical wave. If the back and forth movement is happening in an entirely different plane, that's like an electromagnetic wave.