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 knobs/dials too) on Shabbat (the Jewish Sabbath), as part of the precautions taken to avoid violating the halachic prohibition against cooking on the Sabbath by stirring the fire.
An unblech, or K'Deira Blech (water blech), is also used to heat up pre-cooked food on the Sabbath, but utilizes different halakhic mechanisms than a standard blech. An unblech consists of a shallow metal pan filled with hot water and covered by another metal pan, and thus is akin to a pot of warm food for halakhic purposes. As such it may be more flexible than a standard blech for halakhic purposes. However, the temperature of an unblech is limited by the boiling point of water and is not as hot as a typical blech.