Imagine you have a best friend who loves playing with you, but one day your friend doesn't want to play the same things as you anymore. Your friend still loves you, but you two just don't agree on how you want to play. This is kind of like what happens with separated brethren.
In Christianity, there are different groups of people who believe in Jesus and follow his teachings, but they don't always agree on everything. Some groups may have different ideas about how to worship or interpret some parts of the Bible. When groups like this split up and form their own churches, they become what is called "separated brethren."
Just like how you can still love your friend even if you don't always agree on how to play, Christians who belong to different churches can still love and respect each other even if they have different beliefs. Sometimes, people from different churches can even come together and learn from each other, just like how you and your friend might still learn from each other even if you don't always play together.