חת"ס is an abbreviation for חתם סופר, which stands for חידושי תורת משה סופר ("Innovations of Rabbi Moshe Sofer"). The Chasam Sofer, Rabbi Moshe Sofer (1762–1839), was one of the most prominent rabbis and halachic authorities of the 19th century. He served as the rabbi of Pressburg (modern-day Bratislava), and his rulings and teachings have had a lasting impact on Jewish law and thought.
Rabbi Moshe Sofer was a staunch defender of traditional Judaism and opposed the reform movement of his time. His famous saying, "חדש אסור מן התורה" ("Innovation is forbidden by the Torah"), became a guiding principle for many in maintaining tradition in the face of modernity. His works include responsa (questions and answers in halachic matters), commentaries on the Torah, and halachic writings.
The Chasam Sofer’s influence continues to be felt, and his descendants and students became leaders in many Jewish communities.