In
comics lingo, gutter is the term for the white space between two
panels. It may seem like a little thing, but the gutter is perhaps the most important element of comics.
Sometimes, when I'm preaching the comics gospel to non-believers, I bring up the point that comics as a medium can do things that other mediums can not. For some reason, I am almost always challenged on this point. After all, they say, what separates comics from books or movies? It's simple, really. Comics have interactivity to a much larger degree than other mediums. The beauty of comics is that ultimately, nothing happens in them without the reader. They are simply a series of still pictures placed side by side. But in the gutter, comics come alive.
Our brains are wired to fill in the gaps in our perception. This is called closure. And in between every set of panels in a comic, closure happens. It is up to the reader to determine exactly how Picture A became Picture B, and it's a different process for each reader.
This makes the gutter a very powerful tool for the skilled writer and/or penciler. A writer could never think up a situation or penciler draw an event that makes sense to or resonates with everyone. But if they only present a before and after, than every individual who reads it will make up their own occurence. The trick for a writer is to leave enough in that the reader knows what's going on, but not spoon-feed them the story.
One other application of gutters is to control the flow of time in a comic. Although this is generally accomplished using sound or motion within a panel, wider gutters imply a longer passage of time than thinner ones. Also, by having a character move across a continuous background separated by gutters (a device known as a polyptych), a writer can show their progress through the background over time.
For a much more in-depth discussion of gutters and their importance in comics, or pretty much anything else comics related, read Understanding Comics by Scott McCloud (which, by the way, was my source for this writeup).