Insight is the difference between being told the stove is hot and actually touching it. It's direct experience. In meditation, shamatha (calm-abiding) settles and stabilises the mind; it's what lets you 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.
Insights tend to arrive through gentle, patient investigation, until what was hidden in plain sight stops hiding. Something that was assumed or unquestioned actually gets seen. And in my experience, once it's seen clearly, it's very hard to unsee.