Plates are perforated copper trays that are fixed inside column stills, allowing additional reflux to occur and helping to raise the ABV of the spirit passing through.
How does it work? In all stills, spirit vapour rises up a column still and as it does, comes into contact with the plates above, causing it to condense back into a liquid (see Reflux). This happens over and again, with some liquid dripping back down through the column, while some of the more volatile vapour moves through the perforated holes an up the column.
Due to the rising temperature climbing up the still the falling liquid that drops down is eventually forced back into becoming vapour and attempts to go up though the plates.
This process of being forced from vapour to liquid and back again, as well as the copper contact that occurs creates an end product that has essentially been distilled dozens of times in a single run. This results in a smoother and a much higher ABV spirit.