Can someone axplain to me how important ws th alliance system in starting ww1 ?

The alliances created just prior to WW I (The Great War) were defensive treaties. That meant that each signer of the treaty with the other nation(s) pledged military support when the other nation was attacked. When Germany, for example, told Austria-Hungary it would back them if they went to war against Serbia, Serbia had a treaty with Russia which required Russia to aid Serbia when it was attacked. Because that would now have A-H at war, Germany would have to honor its treaty with A-H and declare war on Russia.