To debase something is to make it corrupt or impure. If your lemonade stand sells “pure lemonade,” you’d insist on using real lemons instead of a mix; using a mix would debase your product.
Debase is often used in the context of two things: coins and people. To debase a coin is to replace some of the precious metal in the coin with metal of lesser value. To debase a person is to corrupt them, often by driving them to perform an immoral act like (gasp!) using the lemonade mix. Just promise us you won’t do it.