What is a mezuzah ?

Thank you for your question. It is a mitzvah from the Torah to write on special parchment the verses that command us to do this, includingthe "Shema Yisroel" sentence. The parchment goes in a casing on every jewish doorpost. You can read more about it here: https://www.chabad.org/library/article_cdo/aid/4297856/jewish/Chapter-11-Laws-of-Mezuzah.htm All the best!