The Article History page shows articles that have previously qualified for one of your newsletter. In order to have previously qualified, it means the article was toggled on as active and eligible to be in your newsletter for a given send. This includes articles that were boosted, super boosted, or marked evergreen.
You can sort your history by send date in order to see which article were active and eligible for a given send.
You can also add articles from your history back into your Upcoming Articles page or to your Evergreen articles. Say there was a great article that didn't go out to everyone, you could add it to your upcoming and then boost it for the next newsletter. Or maybe there was such a great article that you'd love to keep on hand in case you are short on content, then you could add it to your Evergreen articles.