When a large amount of money is needed for any enterprise, from building a factory to funding a corporation to drilling wells in a new oil field, that money is raised from investors usually a large number of them. Commonly, the enterprise raises that money by either selling ownership shares in itself or simply borrowing it. When ownership is sold, the investor gets stock shares. When money is borrowed, the investor gets bonds. Stocks and bonds are both securities.
Investors buy and sell individual securities through brokers, also called securities dealers.
Additionally, mutual fund companies, and other so-called asset management firms, form funds, which consist of a variety of securities. The asset management company buys and sells the securities in a fund, seeking to maximize its value, and it sells shares in these funds to investors directly and through securities brokers. The mutual fund company charges a fee for picking the securities in a fund. In turn, the shareholder is shielded from the risk of investing in individual securities. Principally because the way your parents invested is not how most people do it these days.