In regards to the term gutter, the creator is referring to the space between the borders of the panels in the comic book. It is called this because it looks like the gutter of a house with its two parallel lines. This space is very important as it separates the panels and lets the reader know that we are going form one scene to the next.
In comic books, the gutter is usually white, but that isn't always the case. In other cases the artist has taken leeway on certain pages and mixes the panels together in a seamless set of images for greater impact.
In comic books, the gutter is the thing that separates it from just a page of art and turns it into a sequence of images that tell a story. Without a gutter, the reader would have a hard time discerning the transitions in the story and would make the comic much harder to read.