You can read about impermanence, agree with it, nod along, and still walk round living exactly as if everything's solid and permanent. Insight is different, it's direct seeing rather than agreeing with an idea. It's the difference between being told the stove is hot and actually touching it.
In meditation, shamatha (calm-abiding) settles and stabilises the mind, it's what lets you actually sit still long enough to look at anything properly. Vipassana (insight) is the looking itself, turning that settled attention onto your own experience and seeing what's actually there rather than what you assume is there. Ideally you need both, as concentration without insight just gets you calm, and insight without enough concentration tends to just skim the surface.
What vipassana usually reveals comes down to three things, called the three marks of conditioned existence: impermanence, that nothing actually holds still, suffering, that most of our discomfort actually comes from grasping at things to stay the same, and no-self, that the "you" running the show doesn't hold up once you actually go looking for it.
With insight, it's meant to be checked directly, in your own experience, which is the whole point of practising. Insights also land at every level and with a similar mechanism — something that was assumed or unquestioned actually gets seen, and once it's seen properly, it can't be unseen.
None of this really arrives through thinking harder about it. It comes from investigating experience, directly, again and again, until what was hidden in plain sight stops hiding. That's the practice.