A mutual fund, or investment company, is a corporation which makes investments and passes the gains, losses and taxability along to its shareholders.
Mutual funds provide diversification, since a fund typically owns a few dozen to hundreds of stocks.
Also, by pooling funds from many investors, you can hire full time professional money managers to overseas the investments. Something many people cannot afford to do alone. All funds charge management and brokerage expenses. These expenses vary from fund to fund. It is calculated at the end of each trading day so that people can get in or out daily.