The Vanguard Real Estate Index Fund Investor Shares (VGSIX) is a mutual fund that seeks to track the performance of the MSCI US Investable Market Real Estate 25/50 Index. This index includes companies in the real estate industry, such as real estate investment trusts (REITs) and real estate management and development companies. The fund aims to provide investors with a diversified portfolio of real estate securities, allowing them to potentially benefit from the growth and stability of the real estate market. With a low expense ratio and a focus on long-term growth, VGSIX is a popular choice for investors looking to add real estate exposure to their portfolio.