Something that's knowing is sneakily wise or perceptive. A knowing smile conveys a lot of information — it implies that you know a secret or are in on some background information. Picture Mona Lisa.
A knowing glance tells its recipient that you've got them figured out or that you're aware of something they haven't shared with many people. You can use this adjective to simply mean "having knowledge" or "intentional" too, as when someone makes a knowing purchase of stolen goods. The noun version of knowing is also simple, meaning "the state of having knowledge or being aware."