I was at a conference, proposing what would one day be Miss Match to an editor when I first heard the term "white space".
He was flipping through my proposal and the sample chapters and said, "Oh, you did a great job incorporating white space."
I said, "Thank you." Because I really did not know what else to say. Mostly because I was too busy looking over his shoulder to make sure that I hadn't accidentally given him a blank sheet of paper.
So, I went home and researched. And realized that white space is exactly what is sounds like.
Whenever you are writing - particularly fiction - it's important to keep the story going with lots of dialogue, and not just dialogue, but back and forth dialogue which leads to large white spaces on the page. This looks less intimidating to a reader and so keeps their interest longer.
It's why a lot of times a novel will be a larger size than a nonfiction book - because nonfiction does not have a lot of white space and so looks more intimidating. If you open a book and see one constant paragraph for three pages, you start to get overwhelmed. Publishers have figured this out, so their job is to make your work as inviting as possible.
So, look at your work. Do you have a nice rhythm to your story so that you have some long paragraphs and then several short ones? Do your characters talk back in forth in eighteen-line monologues or do they have a conversation peppered with quick responses?
Listen to the people around you as well. There is a natural flow to conversation and the closer we can mimic that on paper, the more realistic our story will sound and the more easily we'll be able to sell it. ;)