Saussure, like all linguists to the extent that they priviledge spoken forms of language, gives examples taken from phonology. Given the medium used here, it may be best to construct the argument from the viewpoint of a graphic form: the alphabet.

Let's contrast pad, bad, dad and *qad (the asterisk indicates that this possible graphic representation is not in fact found in English, so far at least). The graphs are immediately recognizable as "different" words. The difference between them is based on the alternation in the organization of two basic graphs:
o and |
out of which one makes

d and b by shifting the o from one side of | to the other. Note that none of these units have "meaning" by themselves but that the alternation and differences between them allow us to "mean" as an activity.

Note that this is directly related to what gestalt psychologists had discovered: images (that is symbolically identifiable entities within a human community) are constructed through the organization of relatively arbitrary and often "meaningless" pieces. (See Fraser)