RUT, or the Russell 2000 Index, is a stock market index that measures the performance of 2,000 small-cap companies in the United States. It is a key benchmark for investors looking to track the performance of smaller businesses, which often have different growth patterns compared to larger companies. The RUT is widely used by investors and analysts to gauge the health of the small-cap sector and to make investment decisions. It is considered a barometer for the overall economy, as small companies are often more sensitive to economic changes than larger corporations.