This means your articles should stand the test of time. If you write about Chinese New Year 2011 (that was only a few days ago) then your article will not be relevant anymore in the future. But things like recipes, saving money or home decoration are evergreen because that content is still interesting in many years from now.
You can write about birthdays for a child and if it is done well then it is still evergreen. Many people have children and each year there is a birthday for them. So a hub about what to do on a child's birthday is definitely evergreen because even many years from now people will want to read that article.