It's a casual, informal way to express that you lack knowledge about something.
Here are a few other ways it can be used:
* "Sei la?" - "You don't know?" (asking if someone is aware of something)
* "Non sei la?" - "You don't know?" (more formal)
* "Non lo so." - "I don't know." (more formal)
Context is important when understanding the meaning of "sei la". If you hear it in conversation, pay attention to the tone of voice and the surrounding context to better understand its intended meaning.