Okay kiddo, so you know how there are lots of different Christian churches like Catholics, Baptists, Methodists, and more? Well, one group of churches is called Protestants. These churches started a long time ago in Europe when some people wanted to change some things about how the church was being run. They protested against the old church and started their own groups.
Nowadays, there are lots of different Protestant churches that have different beliefs and ways of doing things. But sometimes these churches come together to do something called communion. Communion is when they share some bread and juice (or wine) and remember Jesus and his sacrifice for them.
So the communion of Protestant churches in Europe means that some Protestant churches in Europe get together to do communion. They are trying to show that they still share some things in common even though they may have different beliefs or ways of doing things. It's like they are all part of the same big family even if they have different traditions.