A broadly diversified portfolio
Each target-date investment holds several low-cost Vanguard index funds to create a broadly diversified mix of stocks and bonds. The year in a target-date investment’s name is its target date, the approximate year in which an investor expects to retire and leave the workforce.
Automatic adjustments
A target-date investment will hold more stocks the further it is from its target date, seeking stocks’ higher potential growth. Stocks also have the highest risk of loss. To reduce risk as the target date approaches, Vanguard’s investment managers will gradually decrease the stock holdings and increase bond holdings over time. Bonds usually have a lower risk of loss, though they also have lower potential gains.

The legal details
Whenever you invest, there's a chance you could lose the money. Target-date investments are subject to the risks of their underlying funds. The year in the investment name refers to the approximate year (the target date) when an investor would retire and leave the workforce. The investment will gradually shift its emphasis from more aggressive investments to more conservative ones based on its target date. A target-date investment is not guaranteed at any time, including on or after the target date.
Bond funds are made up of IOUs, primarily from companies or governments. These funds risk losing value if the debt isn't repaid on time. Also, bond prices can drop when interest rates rise or the issuer's reputation suffers. U.S. Treasury investments and some U.S. government agency bonds are backed by the government, so it’s highly likely that payments will be made on time. But their prices can still fall when interest rates go up. Non-U.S. stocks or bonds have risks tied to the political and economic stability of their country or region. And if the value of the foreign currency falls, the value of the stocks or bonds would also fall.
How a target-date investment works
Whenever you invest, there's a chance you could lose the money. Target-date investments are subject to the risks of their underlying funds. The year in the investment name refers to the approximate year (the target date) when an investor would retire and leave the workforce. The investment will gradually shift its emphasis from more aggressive investments to more conservative ones based on its target date. A target-date investment is not guaranteed at any time, including on or after the target date. Even though target-date investments simplify the investment process, they still require some monitoring to ensure that the portfolio is in line with your current situation. Diversifying means having different types of investments. It doesn’t guarantee you’ll make a profit or that you won’t lose money.