Narration is kind of a word box for a disembodied guide to the comic. Unlike dialogue balloons, it is almost always drawn as a rectangle with no tail. It contains background information to help the reader understand the comic.
Narration boxes are predominantly used in comic books, but they can also appear in other types of cartoons. Use narration boxes sparingly in a comic strip or panel, though, unless they are absolutely necessary for the reader to comprehend the action.