Host features are those which the implementation adds around the native feature set. Host features would include new object types. There are entire groups of objects which collected together provide a host environment capability.
For example, the HTML Document Object Model describes a page as it lives inside a web browser.
The PDF file format can contain a collection of objects and the PDF reader adds others to construct a different kind of object model.
The browser, server or operating system is said to host the interpreter. That is where the terminology of host environments, host features and so on is derived.