Okay, so have you ever heard of a game called hide and seek? Sometimes when we play hide and seek, we might hide something that we're not supposed to have, like maybe a piece of candy that we're not allowed to eat yet. When someone finds that piece of candy, it becomes "contraband" - which means something that is not allowed or against the rules to have.
During the American Civil War, there were things that were considered "contraband" too. One of these things was people who had been enslaved. When the war started, a lot of people who were enslaved ran away from their owners and went to the Union army instead. The Union army said that these people were "contraband" because they were supposed to be property of their owners, but they didn't want to be owned anymore.
So, the Union army took in these people who had become contraband, and they became known as "contraband slaves." The Union army did this because they believed that slavery was wrong and they wanted to help these people who had been enslaved. By taking them in, the Union army gave them a chance to be free and have a new life.
But, not all contraband during the Civil War was people. Sometimes, soldiers on both sides of the war would try to smuggle things that were not allowed or that they weren't supposed to have. This could be things like weapons, ammunition, or even food. If these things were discovered, they would be confiscated and called contraband because they weren't supposed to be there.
So, basically, contraband during the American Civil War was anything that was not allowed or against the rules - whether it was people who had been enslaved and were seeking freedom, or soldiers trying to smuggle things they weren't supposed to have.