Closure is a concept fundamental to artists; it dictates that in the image below, the normal person will 'close' the two parentheses to see the circle that they imply: ( - )
There is some academic controversy about McCloud's use of the word closure, but his broader definition of the term is useful and opens up new avenues of speculation.
Not surprisingly, McCloud relates closure to iconic imagery on pages 90 and 91, stating that cartoonish images facilitate closure because they 'already exist as concepts for the reader.' But he never establishes WHY this would facilitate closure so I find the assertion unconvincing. He illustrates the idea by showing two parallel sequences, one realistic and one cartoonish, of a girl waving at a man who smiles and waves back. But the examples are compromised by the fact that McCloud's 'realistic' example sequence is embellished with a distracting texture that makes the characters look stiff.
McCloud could have made a connection between cartoonish rendering and closure-- when a simple line drawing is used to represent a face, the audience employs a kind of closure to accept it as a face. Of course, he's been over that ground in earlier chapters.
There's a kind of closure McCloud aludes to at the beginning of the chapter: the opening sequence shows a cartoon character walking along, dramatizing how he is only aware of the scenary in his visual range, but using closure to assume the rest. McCloud also refers to a foreign country he's never visited, but assumes it exists because of closure.
I'd call this a 'closure of world view.' For example, in an SF story, we are shown various future inventions, and we imagine a society which uses these inventions and has the advantange of other advancements. In a period piece, we would make similar assumptions. The world view in question is also personalized to the character. A man in a suit carrying a briefcase is assumed to have a job as an executive, and to be financially well off.
Earlier, McCloud indirectly refers to this kind of closure on p. 36 and 37. He shows contrasting pictures of himself as a cartoon character and a realistic character. He then asserts that the former is a better narrator, because he is a 'blank slate.' Says McCloud, 'It would never even occur to you to wonder what my politics are, or what I had for lunch or where I got this silly outfit.' Politics, lunch, his outfit, these are all factors in our world view of the character. And we derive them from a closure based on the realistic attributes of the drawing.
Hence McCloud is stating, perhaps inadvertently, that a realistically drawn character provides greater closure of world view than a cartoon character does. So if one is cartooning a story in which that sense of world view closure is important, a realistic representation might be preferable.