Religious syncretism is like when you mix different toys together to create a new toy. In religion, it means when people mix different beliefs or practices from different religions to create their own religion.
So imagine you like playing with Legos and blocks. You mix them together to build a new toy. In the same way, people may like some parts of one religion and some parts of another religion, so they blend them together to create their own religion.
For example, the ancient Romans mixed their religion with the gods of the Greeks when they conquered Greece. They took the gods they liked from Greece and used them in their own religion.
In some places in the world, people mix Christianity with their traditional beliefs. They may still believe in Jesus but also believe in other gods or spirits from their culture.
Overall, religious syncretism is like making a new toy out of different parts, but instead, it means blending different religious beliefs to create a new religion.