Frequency band
A slice of the spectrum, such as bass, mids or highs, read as one level.
See it in Audio visualizer. Play a song and pick Threads: each thread is one band, bass at the top and highs at the bottom.
In plain words
A full spectrum has hundreds of narrow bars, which is too much to steer a picture with. So tools group the bars into a few wide bands. Bass holds the low thumps, mids hold voices and most instruments, and highs hold hiss and cymbals. Each band is averaged into one level. A visual can then grow with the bass and sparkle with the highs.