It's "Auto H" button for (de)activating Auto Hold (aka Automatic Parking Brake ) function of electronic parking brake (=EPB).

When activated the EPB is engaged everytime the car stops (even when shifter is in N, D or M/S position), and is released immediately when accelerator is pushed.

When Auto hold function is deactivated you can manually engage - disengage the EPB by pushing "(P)" (Parking Brake) button / lever which is located just above Auto H one.