The Dow closed above 40,000 for the first time. The number is big but means little for your 401(k)

Through its long history, the Dow Jones Industrial Average has offered a way for people to get a quick read on how Wall Street is doing. But its importance is on the wane. Because it holds just 30 stocks and uses a quirky way to measure their impact on the stock market, the Dow has ceded its importance on Wall Street to other indexes. So even if the Dow's topping the 40,000 level is cause for celebration, it doesn't inherently mean much. What's more important is that it's one of the examples showing how the broader U.S. stock market is setting records.