Investing in bonds
In order to diversify one's portfolio, it is often recommended to diversify over asset classes (stocks, bonds, commodities, real estate, ...).
Bond ETFs and REITs can be used to obtain fixed income.

The Pimco Total Return ETF (NYSEArca:BOND) is actively managed and has a 0.55% expense ratio.
The Vanguard Total Bond Market ETF (NYSEArca:BND) has an expense ratio of 0.10% and tries to track the Barclays Capital U.S. Aggregate Float Adjusted Index.
The Schwab U.S. Aggregate Bond ETF (NYSEArca:SCHZ) has an expense ratio of 0.05% and tries to track the Barclays U.S. Aggregate Bond Index.