Is there a matching ding on the other side of the rim at about the same place? If yes I'd say it the bike was ridden hard into a pothole with a sharpish edge.
Source - I found an unexpected kerb while riding recently. The front wheel came up in time but the back wheel didn't rise soon enough, and I pinch flatted with cuts ~10mm long, wrecked the tyre, and now have two small dents just like this, but a little longer and shallower.
Personally I'd remove the tyre and gently file off the lip shown in the photo. Look inside for a matching rim. File off no more than you have to. The braking surface should be smooth and not able to catch the brake pad at all. Don't file the outside (round) edge of the wheel, and don't bother trying to fill the gap.
If you're concerned about weight imbalance, file all the dinks out and then put the rim, tyre and tube back in the frame while its off the ground. See if it has a definite "heavier" point that settles lower all the time. Use some clip-on spoke reflectors as weights to balance your wheel again.