It's often used in a playful or sarcastic way, implying that the person being addressed was unaware of something important or interesting.
Here are some examples:
* "Poco no sabías de lo que se estaba hablando en la reunión." (You didn't know much about what was being discussed in the meeting.)
* "Poco no sabías de la sorpresa que te tenía preparada." (You didn't know much about the surprise I had prepared for you.)
The phrase is similar to English expressions like "You have no idea" or "You wouldn't believe it".