I dont know what is a piezo and what it does..... can someone explain me
It's basically a pickup which gives acoustic style sounds. They're installed on the bridge of the guitar. I think you need a pre-amp for them to work though.
Instead of creating a signal by having an object vibrate through a magnetic field, like a standard magnetic pickup does, a piezo pickup creates an electrical impulse when it itself is vibrated. That's why you see them installed under the saddles of acoustic guitars or in the bridges of electric guitars.