Patternism is like following a coloring book. You know how sometimes when you color, there are lines and shapes that you have to stay inside of to make a picture look pretty? Well, patternism is like that, but for people who believe in God's rules. They believe that the Bible tells them exactly how to live their lives, and they want to follow those rules exactly, just like you want to color perfectly inside the lines.
Patternism comes from the idea that the Bible is the most important book in the world, and that it tells us everything we need to know about how to be good Christians. So patternists look at the Bible and try to find every rule and example they can, so they can make sure they're doing everything right.
For example, the Bible tells us to be baptized to become Christians. So, patternists believe that we should be baptized the same way the people in the Bible were baptized. They look for examples in the Bible of how people got baptized and try to do it exactly the same way. They might even argue about things like how deep the pool should be or what kind of clothes you should wear during the baptism, because they want to make sure they're following the pattern they see in the Bible.
Overall, patternists believe that if we follow every rule and example in the Bible perfectly, we'll be the best Christians we can be. It's sort of like coloring by numbers - if you follow each instruction correctly, the picture will come out looking beautiful. For patternists, if we follow the Bible's instructions exactly, we'll be following God's plan perfectly and that will make us good Christians.