XLB is an exchange-traded fund (ETF) that focuses on the materials sector of the stock market. It includes companies involved in the production and distribution of materials such as metals, chemicals, and construction materials. Investors use XLB to gain exposure to the performance of these industries without having to buy individual stocks. The fund is managed by State Street Global Advisors and is designed to track the performance of the Materials Select Sector Index. XLB is popular among investors looking to diversify their portfolios and capitalize on trends in the materials market, especially during periods of economic growth.