What does diversification mean in practice? It means that when you invest in the stock market, you want a broadly diversified portfolio holding hundreds of stocks. For people of modest means, and even quite wealthy people, the way to accomplish that is to buy one or more low-cost equity index mutual funds. The fund pools the money from thousands of investors and buys a portfolio of hundreds of individual common stocks. The mutual fund collects all the dividends, does all the accounting, and lets mutual fund owners reinvest all cash distributions in more shares of the fund if they so wish.