A blech (from the German by way of Yiddish word for tin or sheet metal) is a metal sheet used by many observant Jews to cover stovetop burners (and for some, the cooker's knobs and dials) on Shabbat (the Jewish Sabbath), as part of the precautions taken to avoid violating the halachic prohibition against cooking on the Sabbath.
Read more about Blech: Common Use, Unblech