In general, an embargo is a tool of international diplomacy, whereby some nations agree not to trade with other nations. The idea is to force the embargoed nations to capitulate, as they desperately want or need what the embargoing nations have to offer.
On /tg/, the word "embargo" has a fairly specific meaning: Games Workshop's decision to forbid online resellers from selling outside of Europe.
For whatever reason, GW has the idea that it is reasonable to charge Australians and Americans significantly more for their miniatures than Europeans. Of course, some bright European distributors figured out that they could buy miniatures for their lower prices, and then sell them to overseas customers for less than the GW prices, but still enough to make a profit. GW then said, "stop that or we won't sell miniatures to you." They had some rambling justification that, by letting overseas gamers get miniatures for less money, these distributors were somehow "hurting the hobby," but really, they just want to keep their price discrimination firing on all cylinders.