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It just refers to the weave of the glass fibers--primarily parallel in one direction--in the cloth used. (as oppposed to, for exaggeration, a checkerboard cross-woven cloth.

The idea of uni-directional goes back to the old benchmark--bamboo--that in its natural fiber orientation is unidirectional. Bamboo has naturally tapered fibers that run parallel and don't overlap. If you think about it, merely by rolling anything around a tapered rod, you can visualize the other problem--eliminating much overlap of fibers, parallel or not, at the inner and outer start/end of the wrap. To exaggerate the problem, try to roll a pice of lined paper into a cone shape (a rod being an elongated cone). The related point is to minimize excess material that contributes weight but not "action," which is, essentially, the binding resins and fibers used only to keep in place the working fibers (those forming the rod lengthwise.) Excess materials dampen action and sensitivity. As best I recall, primarily one-direction cloths have been achievable for a long time, and with any fiber material. Processing that eliminated excess resins was equally, if not more, important in refinements of the best blanks of the fiberglass era--Fisher notably. Part of it had to do with the resins used, part with the process of heat and pressure in curing the blank. The technology of cloths and resins was revisited/developed through the graphite era. I'd guess it's pretty standard/easy for any material today. The same concept, by the way, was pursued in bamboo, since its stongest fibers are on the outside; thus efforts to build hollow or fluted rods. I'm not a bamboo person, but as I recall Winston did that the best, so its no surprise they did fiberglass the best also.

Just to revisit this discussion, the current postings of various Phillipson catalogs--even allowing for advertising hype--explain unidirectional fiber very well. I had forgotten, but I guess they pioneered in this technology.