An evergreen Hub, or evergreen content, refers to information that does not change over time, or age.
So, content about current politics would be topical, but the information ages pretty quickly, because things are constantly changing. Content about a President who served 100 years ago would be evergreen, because that information isn't going to change.
The best evergreen content is on a topic people want to know about all the time. So, content about Lincoln or Washington might be more valuable than that about Taft or McKinley.