In most cases, literally an analog volume potentiometer mounted in and controlled by a “treadle”-style footpedal. This allows hands-free control over signal level. There are two main styles available, differing only in their impedance: low impedance is intended for keyboard use or for use in an amp‘s effects loop. High impedance is used with standard electric guitar pickups.
There are some variations on this basic design, such as those using optical cells for changing volume, those with LED indicators of volume level, built-in boost, and so on. In some cases, the pedal may be switchable between a wah pedal and a volume pedal.