X-Outs, or x-out golf balls, are name brand golf balls on which that name brand has been crossed out - usually with a row of X's - and sold at a steep discount to that brand's regular price. A Maxfli, to use an example, Noodle X-Out is essentially identical to a regular Maxfli Noodle. But at some point in the production process, a small blemish of some sort occurs. It could be cosmetic, or perhaps the weighting or sizing is a tiny bit off. Rather than just tossing out these balls, the manufacturers stamp over the name brand and sell them as "Maxfli Noodle X-Outs" (or whatever the brand may be). Most recreational golfers will not be able to tell any difference in performance between X-Outs and their "regular" counterparts.
For more on X-Outs, see our FAQ entry, What are X-Outs, and are They Legal?