The dividends and interest that the mutual fund receives from the stocks and bonds it holds are used to pay returns to investors. These returns are reinvested in the fund for as long as the investor holds it, allowing the returns to compound each time earnings are paid.
This process of compounded earnings, repeated with each asset you invest in, could help you significantly grow your retirement account over the long term. Bear in mind, however, that all investing is subject to risk. No investment strategy, including compounding, can ensure a profit or protect against a loss, especially in a down market.