The wah wah is an effect pedal that alters the timbre and frequency of the guitar signal, creating a very characteristic sound that mimics the human voice in reproducing the onomatopoeic "wah wah" sound.
The first prototype wah wah was produced in 1966 by Bradley J. Plunkett, but already Chet Atkins in the 1950s built a similar instrument and used it on some recordings of the time.
It is probably the "oldest" electric guitar pedal effect in existence and from the 1960s to today it has been used in a wide variety of musical genres, while obviously representing a very important part of the rock sound. Among the historical producers there are obviously VOX and Dunlop Cry Baby. The models produced more recently by Fulltone, Real McCoy and Xotic are also particularly interesting.
