This effect is not an easy one to pull off. It means that the fiction is working on every level--structurally, philosophically, grammatically, narratively--and there are no distractions which draw the reader away from what John Gardner calls the fictional dream. Any distraction, like a wedding band drummer who thinks he's playing in a sixty-thousand seat stadium, will draw the reader's attention, and cause them not to "see" the vision the author is trying to paint.
Almost anything can be a distraction. For example, a writer I knew liked James Joyce a lot, so all of his dialogue used Joyce's two-dash convention, instead of quotes, thusly:
--Heavens, no! James spouted.
--You heard what I said. Renee said.
While stylistically, use of this convention may be avant garde and cool, it still represents one of those bumps which causes the reader to pause and have to think. Other distractions include over-describing, overuse of adjectives or adverbs or problems with narrative voice.
This is why readers and reviewers are so important, and also I think why putting a piece aside for some period of time can be extremely critical to a writer's vision of his own work. Waiting for awhile helps you see the fictional dream in a way you can't when you're too close to it, but more importantly, it helps you identify those choices in your writing that distract from the seamlessness of the whole.
In other words, if your fiction is a rock band, make sure you don't have one of these guys playing drums:
Have a groovy weekend and thanks for stopping by!