The graph shows what signals there are at each frequency as time increases from the bottom to the top.

This has three main features.

There are vertical bands. I guess these are related to the natural frequencies of the structure, which suppresses vibration at some frequencies and allows/enhances it at others.

There are the sloping bands, which shows the increase of frequency of all the harmonics with time, which appears to be fairly linear, indicating that the rotational acceleration is constant.

The energy in the harmonics is highest to the right, which is at the highest frequency. The structure does not seem to suppress these frequencies. All harmonics seem to get larger to a similar power when they reach roughly the same frequency.

I expect the acceleration is proportional to the power. It is measured in dB, and it is a convention in engineering that dB is always a ratio of power, so the bar on the right indicates the colour coding of the power levels in decibels.

Just to complement the accurate comment of Malcolm, it shows this test bed allows a well mastered rotational speed increase, because the sloping bands are so perfectly linear! When doing such measurement on an assembled machine in standard operation, the powertrain RPM increase with time would not generally be linear: anyway all sloping bands would appear homothetic as they are perfectly harmonic (the number of teeth of each gear is invariant). The graph would not be easy to read if the measurement would be taken at constant speed as the gears signatures would show-up as vertical lines, possibly confused with the natural frequencies. So to follow the gearbox vibrational signature you better monitor the start or stop periods, unless you look for possible unsteadiness issues (e.g. instabilities) where constant speed tests are more relevant...

As a whole this graph is usually described as a "waterfall plot". It belongs to the global family of time-frequency representations. In this example, the right setting of the colour scale is essential for maximizing the legibility of the gears signatures!