Hi,
As I can see from the plot attached, this is a time vs frequency plot presented in terms of power(dB). Such plot gives information about the spectrum of frequency components with respect to time. Color map shows the amount of frequency components at given time. For example, in the given plot, frequency component of 4 Hz is higher (shown in yellow) at near 1.5 and 2.5 minutes. One can understand with this representation that, at 1.5 and 2.5 minutes 4Hz frequency was more prominent than other frequency components. In simple words, Power of frequency represents the dominance of given frequency components.
Hope it helps you.
