Wah, in the simplest terms, changes the tone of the guitar with the forward and backward motion of the pedal. A wah pedal can make your guitar sound like....
well....

say this 5 times fast:

wicka wacka wicka wacka

He he...never tried to type out the sound before...Check one out at your local guitar shop...they'll hook ya up. The best way to learn is hands on. Let me know if that answers your question.