Redundancy of course means repeating yourself. As for what that means in fiction, this could be describing things that have already been described, or using two lines to say something that you already got across sufficiently in the first line.
Now, as for redundancy in plot, of course you won't write the exact same scene or conversation twice, but perhaps you have 2 different scenes that reveal the same dynamic between your characters. You enjoy them both, they're both well-written, but they're not both absolutely necessary. They both accomplish the same thing for your story, and are there fore redundant. You must either eliminate one, or combine them into one.
So, especially in revisions, you ask yourself what the scene accomplishes. If it doesn't provide some crucial insight into your characters or some crucial advancement of your plot, chances are it needs to go. And when you have multiple scenes that accomplish similar things, you have some redundant scenes.