Brand X is the stereotypical rival or generic version of the particular branded product you are trying to advertise.
Brand X's product will appear in featureless packaging with 'Brand X' in simple lettering on the front. Your product will sport a catchy name and colourful box.
Brand X washing powder will leave whites dull and lifeless. It will not remove stains. Your washing powder will give dazzling whites and remove 99 common stains on a 40-degree wash.
Brand X stands for every rival brand, but none of them specifically.
Brand X is anonymous, but it is no anti-capitalist tool. It exists to make the advertised brand look good, but avoids giving publicity to your rivals. You can make your product look good, and your rivals look bad, simply by having Brand X represented by the most mediocre version of the product on the market. No rival product is named specifically, so all rivals are tainted equally. No rival product is challenged specifically, so you don't have to address the fact that there is little difference between your product and its competitors.
Brand X is now something of an anachronism, its place taken by the 'leading brand', and more sophisticated advertising techniques. But we all know what it stands for; Brand X is the less desirable alternative, the fear of the unknown, the risk of your breath not smelling quite as fresh if you do not buy the advertised product.