It means that the more you know and learn, the more you realise how less or how little you know. It is said that ignorance is bliss. This means that once you start gaining knowledge you realise how little you know. The more you keep broadening your horizon of knowledge, the more you realise how much more information and knowledge is available. Then you keep learning more and you keep realizing real worth of your knowledge. A frog in the well thinks that what he sees above him is the whole world but once he is taken out he realises how wrong he was. I hope all this explains the phrase in perspective.

This is what Socrates said: "The only thing I know for sure is that I know nothing". 

  Life will always hand us surprises.  We can know things, but  there is always a need to know more.

We need to keep an open mind, and be willing to be taught.

“We can know only that we know nothing. And that is the highest degree of human wisdom.” ―Leo Tolstoy, War and Peace

to Celina

The meaning is very simple.  You know nothing and what you think that you know is really untruth.