I need help with this event. I need a simple summary of it because I don't quite understand what was going on. All I know is that Serbia wanted to be independent so they traded with other countries like France, but Austria-Hungary closed its borders which somehow prevented the trades. I don't understand, please help and thank you.
As the OP mentions, it was about trade, but it was complicated by the interest of various Great Powers in the Balkans. I wont attempt my own summary when there is a simple accurate one available.
Pig War (1906-09) In an attempt to reduce its economic dependence on the Austro-Hungarian Empire, Serbia began (1904) to import French rather than Austrian munitions and established (1906) a customs union with Bulgaria, making tariff laden Austrian goods unsalable in Serbia. Long used to setting economic policy, Austria responded in 1906 by closing its borders to Serbian pork. Serbia refused to bow to Vienna, gained French investment to build new packing plants for international trade, began to order materials from the Austrian rival Germany, and pressured the Austrian-administered provinces of Bosnia and Herzegovina for a trade outlet on the Adriatic Sea. Russia supported Serbia’s actions and war between Austria-Hungary and Russia was averted only because of a German ultimatum (1909) demanding the cessation of Russian aid to Serbia. Serbia and Austria developed a new commercial treaty (1909), but Serbia covertly stirred up trouble among the southern Slavs in newly Austrian-annexed Bosnia and Herzogovina, actions that contributed to the start of WW1
Dictionary of Wars George Childs Kohn, Revised edition 1999