A wah-wah pedal is a type of guitar

Guitar

The guitar is a plucked string instrument, usually played with fingers or a pick. The guitar consists of a body with a rigid neck to which the strings, generally six in number, are attached. Guitars are traditionally constructed of various woods and strung with animal gut or, more recently, with...

 effects pedal

Effects unit

Effects units are electronic devices that alter how a musical instrument or other audio source sounds. Some effects subtly "color" a sound, while others transform it dramatically. Effects are used during live performances or in the studio, typically with electric guitar, keyboard and bass...

 that alters the tone of the signal to create a distinctive effect, mimicking the human voice

Human voice

The human voice consists of sound made by a human being using the vocal folds for talking, singing, laughing, crying, screaming, etc. Its frequency ranges from about 60 to 7000 Hz. The human voice is specifically that part of human sound production in which the vocal folds are the primary...

. The pedal sweeps the peak response of a filter up and down in frequency to create the sound (spectral glide

Spectral glide

A spectral glide is a "modification of the vowel quality of a tone" . Since the vowel quality of a tone is determined by the overtones, spectrum, or timbre of that tone, a spectral glide is a move from one spectrum characteristic of a vowel to another...

), also known as "the wah effect."
The said wah-wah effect was originally intended to imitate the supposed crying tone that a muted trumpet

Trumpet

The trumpet is the musical instrument with the highest register in the brass family. Trumpets are among the oldest musical instruments, dating back to at least 1500 BCE. They are played by blowing air through closed lips, producing a "buzzing" sound which starts a standing wave vibration in the air...

 produced, but became an expressive tool in its own way.