Stock (or common stock) means you are a part owner in the company! Usually there are lots and lots of shares for a single company so owning 1 may mean you are a small owner but still part of the company. With that 1 share you get to share proportionally in the earnings and dividends of the company, so if the company does well you share in those gains. If the company doesn’t do so hot, you run the risk of losing money.
There are lots of ups and downs in the stock market, but it may not affect you as much because we're only recommending a portion of your investments to go into stocks.