Around 55% of Americans report owning stocks. Put simply, stocks are an investment in a company that a consumer may purchase. Investors purchase stocks to earn a return on their investment through capital gains or dividend payouts. Money made and lost on stocks are taxable/deductable if not in a tax advantaged vehicle such as a 401k or IRA accounts (see retirement page for more information).
One may choose to own a variety of types of stocks ranging from an individual stock to an ETF (collection of stocks pre-selected for each fund.) There's a lot of things to keep in mind when picking brokerages such as seeing if there are any trading fees or annual expense fees for
Here are a many popular brokerages that allow the common person to begin trading stocks. Keep in mind, many banks also have their own stock trading brokerages.
Robinhood (Easiest to get started)