Air pressure is plotted on the X axis (vertical) and time in milliseconds (1/1,000 seconds) on the Y axis (horizontal). The top line of the drawing represents a 1-kHz (1,000 cycles/second) tone. It has one complete cycle each millisecond and 1,000 complete cycles in each second.
The second line is also 1 kHz, but it is half the amplitude (up-and-down size) of the first and is shifted in phase (time relation) relative to the first. This means that the second line has exactly the same number of cycles (or peaks) per second, but its size is half the size of the top line, and its peak pressure comes slightly later than the peaks of the top line. The third line represents a 2-kHz tone, which has exactly twice the peaks per second as the first two. Its amplitude is the same as the amplitude of the top line.
Pure tones are rare in nature. Natural sounds can be described as mixtures of many sine waves with different frequencies and amplitudes. The fourth line of the graph above shows such a mixture: the top three lines combined to form a complex wave. This mixture is still much simpler than most natural sounds, which have many more than three pure tones in them.
The three frequencies in this graph are simple multiples of each other, so the sum shows a regular repetition, like the individual sine waves. Such a mixture is called harmonic, and the sound it would make would have a musical quality.
The drawing below represents three sine waves that are not simple harmonics: 1.0, 1.7, and 2.4 kHz are not simple multiples of each other. The sum is quite irregular and would sound harsh and noisy.