Confused with mBreakIt variable. What is it for?

The variable itself doesn't do anything for the class. Craig simply uses it to demonstrate how serialization works. In a high level view of serialization, we could say that when a class is serialized it is put into a certain binary code. Now we send this class across the internet to Bob. He gets the class in its binary code format. When Craig changed his class ( by putting a random variable in there, although it could have been any change), he can serialize it and it won't be in the exact same binary code as Bob's version. When Bob receives the same class but with that new variable, it won't match the old one. So, we can give it a serialization unique identifier (UID) and the class will get serialized with this ID embedded in the binary code. Now we can send and recieve that binary code of that class and as long as the UIDs match, then everything is fine. Watch that last part with this in mind and listen to him carefully and it should make a little more sense. If it's still confusing, just search it in a search engine and read different ways to define it.