Security is just designating that is a type of investment.
-A stock is a security.
-A bond is debt security.
Now with a security you purchase it in the case of a stock, or with a bond you lend your money to someone else.
Or you purchase an existing bond from an existing owner. And you have an actual piece of paper that indicates you are the owner of that investment.
Stocks go up and down with the market and pay you dividends. Bonds promise to pay you back, but along the way they’ll pay you interest. It’s important to understand what you own and why you own it.