Everything that people don't say is an open book to you. You read body language and other tells to know exactly what someone is thinking, even if they are trying to hide it.
Additional Rules: Some Insight powers let you deduce a character using certain qualities. When you roll any skill against an NPC, you can choose one of these qualities and tell the GM what you think that quality is for that character. If you are correct, you gain advantage on that roll. If you are wrong, you gain disadvantage. Once you attempt to deduce a character using a specific quality, you can't deduce that character using that quality again.
Complications: You read some detail incorrectly. At some point later, when you try to deduce, the GM will grant you disadvantage even if you are correct.
Experience: You gain insight experience when you call out a character on a lie they have said, or some truth that they do not want you to know.