Okay kiddo, the Waterloo Declaration is a piece of paper that some very important people signed. These people were leaders of different churches, kind of like bosses of different groups of religious people. They signed the paper to say that they all agreed on some important things about their religions.
One of the things they agreed on was that they all believed in Jesus and they all followed the Bible, which is a really important book in their religion. They also agreed to work together to spread their beliefs and help people who needed it.
The reason this was a big deal is because sometimes different religions disagree with each other and it can cause problems. But by signing the Waterloo Declaration, these leaders showed that they could put their differences aside and work together for the common good.