Onomatopoeia

If you think a dog goes "woof woof" or "bow wow", you're wrong. Dogs in Japan go "wan wan". Apparently even animals have their own language differences in other countries. Onomatopoetic words are those that imitate natural sounds, or are references to actions. In the japanese language, they are often in pairs of the same sound. Examples are "bera-bera" (talk non-stop) and "peko-peko" (very hungry). There are hundreds of them around, and it is used a lot on the streets and even on TV commercials, but you'll almost never read about them in Japanese textbooks.

Another example:
"Pika Pika": flickering light or flash
"Chu": the sound a mouse makes. think "squeak"
Put them together and you get "Pika Chu"!