The wah, or wah-wah, is an effect achieved with a pedal called an “envelope shifter.” The pedal works like a super tone knob and dramatically changes your tone from dark to light. The change is initiated from the player's foot rocking back and forth. The most popular pedals are made by Dunlop and Vox, although there are others made by such companies as Morley or Boss. The name “wah” comes from the sound made when you rock your foot back and forth. You'll have to hear it to really understand the sound. Hendrix popularized this pedal, and it has become a staple of guitar players everywhere.