We use it to detect beats and make visualizers react accordingly.
Every sound resonates at a certain frequency. When creating a visualizer we analyze your audio and detect where the beats are in the song. The frequency range within which we look for these beats is called beat frequency.
For example, if your beat frequency is set to the 20-200Hz then our algorithm will look for beats in that frequency range (bass beats).
You can find beat frequency settings in the Customization tab when editing visualizers.