Bandwidth is the range of frequencies in which an electronic circuit (such as a filter, amplifier, or mixer) effectively operates.

Think of bandwidth as the "diameter of a pipe" for frequencies. A wide pipe (wide bandwidth) lets through a large range of frequencies, while a narrow pipe (narrow bandwidth) lets through only a very specific range.

By convention, bandwidth is measured between the points where the signal level drops 3 dB below the maximum level.

These points are known as "cutoff frequencies".

For example, the bandwidth of human hearing is approximately 20 kHz (from 20 Hz to 20 kHz). In an equalizer, the bandwidth (often controlled by the "Q" parameter) defines how "wide" or "narrow" the bell of your adjustment will be, determining how many neighboring frequencies will be affected.