The Campbell diagram is a mathematically constructed diagram used to check for coincidence of vibration sources with natural resonances. The form of the diagram is like a spectral map, but the amplitude is represented by a circular or rectangular plot, the larger the amplitude the larger the circle or rectangle.
In this particular example the even engine orders are plotted from 1E to 10E and a resonance is seen at approximately 120Hz. There is a resonance at 260Hz excited by 4E and another at approximately 500Hz that is excited by 6E.
If this was a 4 cylinder 4 stroke engine 2E would equate to the firing frequency.
In this particular example, the smallest circle equates to a sound pressure level of 35dB(A) and the largest to 65dB(A).
The Campbell Diagram is often paired with a 2D Watefall Plot, such as the one you see here.