Okay, so there was this guy named Merriman who lived in Maryland. During the Civil War, he supported the South and helped them out. But the President of the United States, Abraham Lincoln, didn't like that because he wanted the whole country to stay together.
So, Lincoln told the army to arrest Merriman and lock him up in jail. But Merriman didn't think that was fair because he hadn't done anything wrong according to the laws of Maryland. He asked the court to help him, but Lincoln didn't want the court to get involved because he thought he was doing what was best for the country.
So, Merriman's lawyer went to the court and asked them to help. The court said that it was important for them to protect people's rights, even during the war. They said that Lincoln couldn't just arrest people without a good reason, and that he couldn't ignore the laws of Maryland.
But Lincoln didn't listen to the court. He said that he was the President and he knew what was best. He kept Merriman locked up in jail without a trial for a few months until he finally let him go.
Overall, Ex parte Merryman was a big deal because it showed that even during a war, the courts had a duty to protect people's rights and make sure that the government didn't abuse its power.