The Bible teaches that thousands of years ago God gave his people, the Israelites, a set of rules to live by. The rules showed the Israelites how God wanted them to live. When the Israelites followed the rules they lived God's way.
However God's people soon discovered that on their own they could not keep the rules; they kept on making mistakes and getting things wrong.
God's rules showed the Israelites where they were making mistakes and getting things wrong but the rules themselves didn't actually change the way they were living.
People who study the Bible teach that just trying to follow God's rules is like looking in a mirror. A mirror can show something is wrong, but it can't help put it right - it can show your face is dirty but it can't wash away the dirt.
The rules not only showed the Israelites how God wanted them to live, they showed them they needed God's help to live his way.