Question

what do all the metal pieces get polished before the experiment

Answer 1

because they contain a layer of unwanted impurities that slows down the reaction
Hence they have to be polished before experiment to remove the impurities and make the reaction more reactive
hope this helps you

Answer 2

because when a metal is kept in open air it react with substance present in air and a layer is formed on metal so to remove the layer metal is polished before experiments