It means that they are guppies that have not been inbred enough to fix any particular color or tail patterns that we often associate with modern guppies. All guppies are "pure" in the sense that they are derived from Poecilia reticulata that were once wild. Pure endlers are simply endlers, Poecilia wingei, with no guppy in their background. The things that pet shops call endlers are most often a cross between a guppy and an endler. To my way of thinking they are a pure waste of tank space but to each his own.
The difference between a modern guppy and a wild guppy has nothing to do with crossing outside the species, it is more a case of selecting the fish that show the characteristic that you find interesting and want to preserve in the next generation.