You asked what the power spectrum plot means. Your plot shows multple traces for each of the three conditions: Healthy, Bearing Fault, and Broken Bar Fault. The traces within each condition are similar, but the three conditions are different. You did not explain the meaning of the infrequent dots along the traces, so I cannot address them.
Any signal can be represented as a sum of sinusoids, if you choose the amplitude and phase of each sinusoid correctly. The power spectrum plot shows, on the vertical axis, the power (i.e. amplitude squared) of the component sinusoids, at each frequency (horizontal axis). If you sample the signal at 240 Hz, then the component sinusoids will have a maximum frequencies of 120 Hz, as shown in your plot.* The power is shown in decibels on your plot.
The change in the shape of the spectrum from healthy to the fault conditions is a reflection of the change in system dynamic behavior when there is a fault. The healthy traces show a modest local peak around 80 Hz. The fault data do not show such a peak. The power when there is a bearing fault does not vary as much across the frequency range as the power varies in the healthy and broken bar fault conditions. The power at low freuencies (0-5 Hz) is lower in the bearing fault condition than in the healthy and broken bar fault conditions.
*The sampling rate could have exceeded 240 Hz, in which case the component sinusoids would go up to a frequency exceeding 120 Hz, and the graph could have been clipped on the frequency axis, to an upper limit of 120. I don;t know how the graph may have been manipulated.
