Pick microphone or file as the source, grant permission or load a file, then press Start. The spectrum animates live and you can change the FFT size while it runs.
Each bar is a frequency band, with low frequencies on the left and high on the right, and the bar height is how much energy the signal currently holds in that band. Bass shows as tall bars on the left, cymbals and sibilance on the right.
The FFT size sets how many frequency bands are computed. Larger sizes give finer frequency detail but respond more slowly to change, while smaller sizes react instantly but blur nearby frequencies together.
No. The live input is passed to an analyser node purely to compute the display and is never stored or transmitted. Stopping the analyser releases the microphone straight away.