I am assuming you mean width at the gunwale and width at the water line. In tandems I wish the would have width at gunwale paddling stations.
Ok back to what you want to know.
Width at the water line give a little bit of information about the hulls interaction with the water. Narrower mostly tells you faster/tippy. Wider tells you slower/stable. But this is subjective, general, and only a little slice of the hulls design.
Width at the gunwale can tell you many things some of which are how far you have to lean to the side to move the paddle, volume the hull will hold, and other things.
Some hulls are specified with 3 values:
Depending on hull design, the gunnel width will be less than the maximum width, and may even be less than the waterline width. Many flatwater hulls have tumblehome, where the gunnel width is less than maximum width, to make it easier to reach over with a paddle.
Some whitewater hulls have considerable flair at the gunnels, for reserve stability.
Stripperguy nailed it, above, except for adding the more formal "molded" width which, at rails, is the laminate rather than the outer edge of the outwale. He is a casual guy who would be expected to shorten or drop formal appellations.