A mutual fund is a publicly traded pool of money managed by an investment firm that tries to hold to certain risk / return parameters, with the goal of benefitting anyone who invests. They include a lot of different securities (aka they’re diversified). That makes investing in a fund generally safer than investing in a single security — external factors, like market shifts, are likely to affect each security in a different way, and not all at once.
Mutual funds are also relatively liquid, meaning you can get your money out of them pretty easily. Mutual funds are traded once, at the end of the day (unlike exchange-traded funds, which can be traded throughout the day like individual stocks).