Objective reality is a set of basic observances made about the world that society as a whole has agreed to support as their version of the events that happened. If one man sees a dragon flying through the air, well, he’s out of luck, because no one else saw it and it is therefore “not real.” There’s also the fact that it would be rather improbable for a dragon to physically exist given their size and the difficulties involved in breathing fire.
We all agree on the objective reality because that’s the only way that we as a people can exist. If we continually put our subjective realities forward, we’d never be able to accomplish anything, as we’d be too busy fighting over the differences in what we believed. Oh, wait.
But that isn’t the point of this. The point is that the way we perceive things changes how we interact with the world. Donna was able to notice that something was a bit off with her world because of the way time kept jumping around. She then promptly chose to ignore it, partly because someone was messing with her mind (she saw the Doctor, and then she forgot), and partly because she had a good life set up. Two cute kids, a nice house, a gorgeous man who wouldn’t ever contradict her… who would walk away from that?
Memory and subjective realities are funny things, especially when time travel is involved. This is one of the episodes that you should watch again after seeing several more with River Song.