it was a trigger that showed that there were two competing powers in Europe: Russia and Austria-Hungary (and their allies).
Franz Ferdinand was killed by a Serbian separatist and the Austrians wanted to see him punished. On the other hand, Russia supported Serbia is saying that Austrian demands ( to be allowed to search for the culprit inside Serbia) were too much.
This lead to a direct conflict between the two powers (Austria and Russia) and further lead to the war.