Imagine you and your best friend are playing a game of hide and seek in the park. You both agree to hide in different spots and the first one to find the other wins. You hide behind a big tree and wait for your friend to come find you. As you wait, you hear some rustling in the bushes and suddenly your friend jumps out from behind the tree, surprising you. Your friend says they found you first and won the game, but you disagree. You say they cheated because they didn't follow the rules and instead of finding you, they just jumped out and scared you.
This is kind of like what happened in Germany after World War I. Germany had been fighting against other countries in the war, but eventually they lost. Everyone in Germany was very sad and mad that they had lost the war. Some people in Germany started saying that they didn't really lose the war because of fair fighting, but because of secret plots and betrayals from their own government. They believed that their government had given up too easily and had not fought as hard as they could have. They blamed their government for "stabbing them in the back" and causing them to lose the war.
This belief, called the "stab in the back myth", became very popular in Germany after the war. Many people believed it, even though it wasn't true. It helped to create a feeling of resentment and anger towards the government and other groups, which led to some very bad things happening later on in history.
So just like in the hide and seek game, some people in Germany thought they were betrayed and didn't lose the war fairly, even though that's not what really happened. The stab in the back myth is something that we should always try to avoid believing because it can cause a lot of harm and hurt.