Use the phrase "What that means is that ___" to explain something that was just said. For example, imagine a science teacher explaining a new idea to her class:
They are symbiotic organisms. What that means is that each one depends on the other to survive.
You can leave out the word "that" if you want:
There's a hold on your account. What that means is you'll have to go down to the registrar's office and pay up those overdue fees first.