Judgement is basically knowing a bunch of things but having a good idea of which things are more important than others, and especially knowing which things are worth spending time on any which aren't.
It's like when I see someone talking about financial options and they put everything on the board at once: what are puts and calls, what's an iron butterfly and other strategies, do they need to know the payoff diagram, what about black scholes, what are the Greeks, when should I exercise, and so on. They are all things that a professional option trader knows, but when you're a pro you reduce your cognitive load because you know what actually matters and you are not juggling all these concepts at the same time.
It's also the source of frustration when interviewing. Say you've written CPP for many years, you're probably not prepared to answer questions on all corners of the language, even though you're an expert in some sense. Someone relatively newer might be better at that task, because they're thinking a lot about everything, including things they haven't decided are unimportant yet.