In a functional test, you test the features that the system offers as a whole, seen from the end user’s perspective.

In the previous millennium such a test would be planned by a person with domain knowledge, then executed prior to every release by testers performing manual operations based on written instructions.

A more contemporary strategy is to have the person with domain knowledge manage a team of developers, who are actually implementing the tests as code, and then give the software developers access to execute these test in their production-like environments.