Think of it as an investment within or from another investment... It is derived from the primary product.
So a share of Apple stock is a
Security you can trade. But you can also purchase the option to buy/sell a share of Apple at a given price within a given time. This option is derived from the primary security, Apple in this example, and is a derivative investment.
This works the same way in debt instruments (bonds, mortges, etc). A loan is a primary instrument but if we break that loan up into segments and bundle it with other loans to create a CMO (collateralized mortgage obligation) the CMO is a derivative of the initial loans.
It's a tricky topic; hopefully this helps.