An insight check tells you if you THINK the other person is lying or not.
I will try and explain
@zakael19's point, as I understand it.
Let's suppose that the player declares for their PC, "I observe the NPC's body language, dryness of the mouth, sweat, etc. Do they seem to me to be lying?"
The GM, who has read the rules -
Your Wisdom (Insight) check decides whether you can determine the true intentions of a creature, such as when searching out a lie or predicting someone’s next move. Doing so involves gleaning clues from body language, speech habits, and changes in mannerisms. - responds "OK, roll WIS (Insight)." And the GM sets a DC.
The player makes the roll, adding their appropriate modifiers. If the GM has told them the DC, they can see whether or not they succeed. If the GM has kept the DC secret, they can still see whether they rolled high or low and hence hazard a guess as to whether or not they succeeded.
If the GM says, "The NPC seems to you to be lying", and the player knows or reasonably believes that they succeeded, then their suspicion is confirmed. They can confidently declare actions for their PC on the premise that the NPC is lying.
But what if the player knows or reasonably believes that they failed? If the GM says, "The NPC seems to you be sincere", is the player allowed to nevertheless declare actions for their PC premised on the NPC being a liar? If they do, doesn't that contradict the GM's narration of the outcome?
And suppose that the GM says, "You don't get any sense of whether or not the NPC is lying or sincere", then the same question arises - is the the player allowed to nevertheless proceed on the basis that their PC is suspicious of the NPC's sincerity? If they are, what was the point of the check? What meaningful contribution did it make to the fiction?
The moral of the story - speaking now for myself rather than trying to explain my understanding of
@zakael19 - is that the mechanics don't seem to work in any sensible way
if the player is not bound in some fashion by a failed roll. Because it just becomes a free test to try and get extra information from the GM. Unless the GM takes a completely different approach - eg on a failed check, the GM narrates "The NPC is incensed by you staring at them!" - to enforce some sort of consequence for failure. But I don't get the sense that that sort of failure narration is very common among 5e D&D players.