The archive is the backfill of all the stories in all of your feeds. If you were to click on All Site Stories, you could scroll, and when you’re done scrolling, that would be every story from every feed ever published.
Premium archive accounts also get the luxury of having NewsBlur proactively backfill all the content if possible. A lot of blogs support this, and there are two different methods that NewsBlur uses to find all the previous stories written. This can be super handy for when you want to search stories or share stories from the archive. Remember, related stories for premium archive users go deep in your archives so you can find stories related no matter how old they are.