VTBIX is a mutual fund that seeks to track the performance of the Bloomberg Barclays U.S. Aggregate Float Adjusted Index. This index includes a wide range of investment-grade, taxable, fixed income securities, such as government, corporate, and mortgage-backed bonds. The fund is managed by Vanguard and aims to provide investors with a diversified portfolio of high-quality bonds while keeping fees low. VTBIX is suitable for investors looking for a low-cost, passive approach to investing in the bond market.