This bar is a real-time spectrum analyzer. It calculates the frequency spectrum of the currently playing audio, and displays the result as a dynamically updated plot in which the horizontal axis is frequency in Hertz, and the vertical axis is amplitude in decibels. To show or hide the bar, use View/More Bars/Spectrum Bar. For static analysis, use the spectrum command (Audio/Spectrum) instead.
The real-time analyzer's behavior and appearance are configurable, via the parameters on the Spectrum page of the options dialog. Some of these parameters are also accessible from the spectrum bar's context menu. The real-time analyzer supports various plot styles, and can optionally show peaks. Note that while the real-time and static analyzers have similar parameters, they're completely independent of each other and can be configured differently.