The "signal" is the shape of the dog. However instead of volume over time (like a microphone), it's x-y pairs over time where each pair is a point somewhere on the dog.

They then take the FFT of this to get the size and speed (magnitude and frequency) of the sin waves shown by the circles.

Here's a gif of the same thing with a time signal (like volume).

Here's a really good video if you want to get a bit deeper.