PropertyChanged.Fody is an Intermediate Language (IL) weaver that will automatically add code to your model properties at build time. It will—more specifically—inject a call that will raise the PropertyChanged event whenever a setter is called. It also takes care of property dependencies—if I change the FirstName property, the FullName read-only property will also get a PropertyChanged event. Before PropertyChanged.Fody, you would have had to write this code manually.
It's all explained in more detail in Chapter 2, Building Our First Xamarin. Forms App. Have you read it yet?
