Many of us have the habit of saying something in a rather long-winded way and then "so what I mean is.." and giving a summary of what we've just said. This can of course be very useful when explaining something complicated or difficult to grasp.
그러니까 is the perfect word to use. In this context, 그러니까 can be translated as “so...".
After 그러니까, you summarize what you said before or give a more detailed explanation.
However, in Korean, 그러니까 can also be used to mean “you mean...” so can be used when you are checking with the other person on what he or she has said.
그러니까 이거 저 준다고요?
= So, are you giving this to me?
그러니까, 벌써 다 했다고요.
= I mean, I already finished it.