Got it. This plate should be somehow connected to ground, and the breadboard should be positioned over the plate (but unconnected to the plate itself).
So, this will help to get rid of some unwanted noises... Good, because I have bought the breadboard for trying out high frequency circuits indeed, thanks for the reply!
But is it okay to just plug the aluminium plate into the breadboard ground rail then (which is connected to the Arduino GND pin)?