This is based on Figure 2.21, which is shown faintly in the background.
There is a horizontal frequency axis marked with six equally spaced markers labelled ‘subcarrier frequencies’. Centred on each frequency is a faint wave shape with a high positive peak centred on the marked frequency, falling away sharply on either side in ripples of decreasing amplitude. Because the frequency markers are close together, there is a lot of overlap of the wave shapes, so that as the wave shape falls away on either side of the peak, at a value of about three-quarters of its peak value it intersects the wave shape centred on the adjacent subcarrier frequency. Each subcarrier peak coincides with zeros on all the other waveforms it overlaps with.
A series of six tall, thin, adjoining rectangles is shown much more boldly over the wave shapes. Each rectangle is centred on a subcarrier frequency, and each is as tall as the peak of the wave shape underneath. Each rectangle is wide enough to encompass the peak of the wave shape underneath, and the wave as it drops away from the peak and intersects the adjacent waveform.
The rectangles are labelled ‘subchannels’, and the width of a rectangle is labelled ‘subchannel width’. The total width of the six rectangles is labelled ‘overall bandwidth of multiplex’.