The Farrer hypothesis is about how the books in the Bible were written. It says that the Gospel of Mark was probably the first one written because it is the shortest and it is very similar to the other two Gospels, Matthew and Luke. The idea is that Matthew and Luke used Mark’s Gospel as a template when they wrote their own versions.
Think of writing a story like building a block tower. Mark started with a small tower and then Matthew and Luke used that same block tower to build their own towers using some of the same blocks and also adding some new blocks.
Now imagine you’re playing with those block towers and you accidentally break one of them. You could try to fix it by taking apart the broken blocks and putting them back together in a different way. This is kind of like how the Gospel of John may have been written.
So, the Farrer hypothesis is a way of understanding how the different books in the Bible were written and how they are related to each other using the analogy of building a block tower.