A benchmark is a standard index used to measure portfolio performance. Comparing your returns to a benchmark tells you whether your investment decisions are adding or subtracting value.
Excess Return = Portfolio Return - Benchmark Return
If your portfolio returned 15% and the S&P 500 returned 12%, you outperformed by 3%. If you returned 8%, you underperformed by 4% and would have been better off holding the index.
AllInvestView compares your portfolio against benchmarks (S&P 500, MSCI World, etc.) on your analytics page. See also Alpha.