But what is a mutual fund? It's not complicated. A dictionary definition of a mutual fund might go something like this: a single portfolio of stocks, bonds, and/or cash managed by an investment company on behalf of many investors.
The investment company is responsible for the management of the fund, and it sells shares in the fund to individual investors. When you invest in a mutual fund, you become a part owner of a large investment portfolio, along with all the other shareholders of the fund. When you purchase shares, the fund manager invests your funds, along with the money contributed by the other shareholders.
Every day, the fund manager counts up the value of all the fund's holdings, figures out how many shares have been purchased by shareholders, and then calculates the Net Asset Value (NAV) of the mutual fund, the price of a single share of the fund on that day. If you want to buy shares, you just send the manager your money, and they will issue new shares for you at the most recent price. This routine is repeated every day on a never-ending basis, which is why mutual funds are sometimes known as "open-end funds."
If the fund manager is doing a good job, the NAV of the fund will usually get bigger -- your shares will be worth more.