While I would not recommend bond investing to an aggressive 23-year old investor, diversification serves a major purpose in most investment portfolios. Bonds are another class of investment that will lower risk and help your return in a long-term investment horizon.When investing in bonds it’s best to spread your risk over a series of different maturities, while maintaining an average maturity of your liking in your portfolio with a bond ladder.

If you were to buy equal amounts of bonds due to mature in one year, two years, three years, four years, five years. This insulates your risk of the bond market being out of favor–your average maturity is 3 years (somewhat like dollar cost averaging with stocks).
The next year your first set of bonds comes due, you would reset the ladder by putting the money into new 6-year notes. Your portfolio would remain at an average maturity of three years.The year after that, when the two-year notes matured, you would buy more 6-years, and continue to do so whenever a note matures. That would always keep the average maturity around three years.
You can do this with any kind of bond with any set of maturities you like.