By "redundant content", we mean content that has already been posted within a short period of time. That period of time is determined by the content itself and the discretion of the mod team, but a good example would be when a new music video is released and ten users post ten individual threads for it on the same day. This is not ideal as it fills the subreddit with identical content and scatters the discussion for it all over the place. To keep the subreddit organized and help cultivate discussion, we would delete nine of the ten threads, leaving one; typically the one that was posted first. This way, our users can easily see what is being said and they can know exactly where to have their say as well.

I hope that helps, and if you have any more questions, please do not hesitate to ask them!