It's when you search for, or discover, a meaning that's implied rather than actually spoken. So if a friend tells you they can't meet you because they are sick, but you don't believe them, we might say 'she told me she is sick but reading between the lines I think she's actually meeting another friend instead'

NOT a secret message. Simon's response is best. For example, imagine you ask someone out, and they respond by saying they are too busy this friday, then list a bunch of tasks they have to do that really don't sound that important. If you 'read between the lines' what they are really saying is not that they are busy, but that they don't want to go out with you.

It means to understand what is being communicated without it explicitly being said. It is kind of like "to put two and two together." They are similar in meaning.

To make sense of a situation that is not clear

Hi Lilith, To "read between the lines" means to see or understand things that are not clearly visible, in many cases this could be a secret message!