what does this button used to in Toyota Prius 2012 ?

Look in your owner's manual. I like the 9 hours ago anonymous answer.

It releases an oil slick behind your car making chasing cop cars spinout

Button is for disabling the Pedestrian Alert System or Vehicle Proximity Notification System.

HI so consult with a toyota dealer.

It mutes some audio device: either your car's internal sound system (for talking on the phone or out of the window at a toll booth), or shuts off the pedestrian-alert beeper on the front of your Prius while driving silently on battery power with the engine off.

that's the eject button

Under no circumstances press that button. That is for your ejector seat