The Article History page shows articles that have previously qualified for one of your newsletters. In order to have previously qualified, it means the article was toggled on as active and eligible to be in your content pool for a newsletter for a given send. This includes articles that were boosted, manually entered, and all other content that was in your upcoming content pool from your sources.
You have the ability to sort your history by send date in order to see which article were active and eligible for a given send: there is an option to view all past articles as well. Additionally, you can also sort your history by Source, Section, No Image and Image Warning.
In addition to viewing qualified content, you can also add articles from your history back into your Upcoming Articles page. For example, there was a great article that didn't go out to everyone, you could add it to your upcoming content pool and then boost it for the next newsletter to ensure all subscribers see it. 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 upcoming content pool for the next send.