Other’s have mentioned the quote “to clarify, add detail”. But what Tufte says afterwards is equally, if not more, important. “Clutter is a failure of design. If the information is in chaos, fix the design, don’t throw out information.”
I think far too often we get too strongly married to our own ideas, especially after spending significant amounts of time and effort on them, that we can’t step back and see that it’s not working. That’s one of the reasons I opened all of my phone designs together in Preview to see how they related. At the same time, I know I have also been guilty of not being fully satisfied with a design, and taking it one step too far. That one extra step can completely ruin a piece, and in the art world can be irreversible without the handy “command + z” option.
Whether you’re adding detail to clarify, because you think it’s necessary or because you’re not yet satisfied with a design and are exploring options, you can clutter your work. If the detail is necessary, it might mean shifting around what you already have rather than trying to incorporate it in around what’s already there. I think this is an area where newer designers struggle, focusing more on the work already put in rather than the success of where they currently stand.