A toy is said to be shortpacked when it is shipped in signifigantly lower numbers than the rest of its release wave.
A toy that is shortpacked is typically harder to find than other toys from the same assortment, since there are fewer of it produced. This rarity also makes it attractive to scalpers. If a shortpacked toy is also a highly desirable one to fans and/or collectors (for instance, if it represents a popular character who is rarely produced as a toy), then it can become extremely difficult to obtain.
Sometimes, toys which are shortpacked in one wave are released in greater numbers in a later wave, which tends to really piss off the collectors who bought from the scalpers, and delight those who didn't.
While inspiring similar feelings for many fans, shortpacking should not be confused with chase figures which are aimed exclusively at collectors and generally and intentionally much rarer (a shortpacked toy may be 2 per case while a chase figure may be as little as 1 per every 2 cases)