Okay, let me explain it to you in simple terms.
A long time ago, there was a war in Iraq. Some countries, like the US, Britain, and Australia, went to Iraq to fight this war. They wanted to get rid of the leaders in Iraq who were doing bad things.
But many people didn't agree with this war. Some people thought that there was no good reason for the war, and that it would only make things worse. They believed that war would cause lots of problems, like hurting innocent people, damaging cities and causing people to lose their homes.
There were many protests and demonstrations all around the world against this war. People wanted their governments to stop the war and find a peaceful solution. Some people believed that by going to war, more problems would be created than were actually solved.
So in short, people opposed the Iraq war because they thought that it was wrong and would cause more harm than good. They wanted their governments to find a peaceful solution rather than going to war.