Ideally, features are areas of your product incorporating a range or functionalities or user stories.
What a test IO Feature can contain:
User stories to cover / functionalities to test
What is considered a bug and what is intended behavior
Known issues for this feature
For an e-commerce website, the feature set might look like this:
Registration & Login
Filters and Sorting
Product Detail Page
If you are still not sure how to proceed, please do not hesitate to reach out to your Customer Success Manager, or check out the "What are Features?" FAQ article for more information.