Feature files are the files that describe certain behavior of your application per feature in a scenario-based way. Each determined scenario describes the state and expectation you have of the system.
Note that this documentation, nor JOSF dictates on how to write good Gherkin. There are many other resources on this subject found on the internet that describe this process best. A good starting point would be https://cucumber.io/docs/gherkin/reference/. Note that this documentation references to Cucumber. Cucumber is a different test framework, which requires a technical implementation of step definitions (written in Java). In JOSF, you can implement the step definitions yourself, since JOSF is low-code.