To know something means to have a piece of information or a certain understanding of something. If someone tells you their phone number and you remember it, you know.

There's a reason you can find the word know in the word knowledge — to know is to have knowledge about something, to be familiar with an idea, or to recognize someone. If you're "in the know," you have information that is only known to a select group of people. But be careful — to know someone in the Biblical sense means that you're having sexual relations with them.