It is a plot of the short time Fourier transform of the signal with a window and it is a FUNDAMENTAL RESOURCE to identify the notes.
Our window length: 5000 It is chosen to achieve a trade-off between the correct frequency value measurement and the minimum overlap of samples in time domain
3D plot - Why? x axis:window in time domain y axis: frequency values Color:Yellow: frequencies with the highest power Blue: frequencies with the lowest power