The Northern Subject Rule is basically a way of speaking that some people use in parts of the world where they speak English. When you use the Northern Subject Rule, you talk a certain way that is different from other parts of the world where people speak English.
In this way of speaking, when you make a sentence, you always use the word "do" before you say the verb. For example, instead of saying "I like ice cream", you would say "I do like ice cream". You're adding this extra word "do" in there! It's like you're saying "yes, I really do like ice cream, it's not just a little bit, I really, really like it!"
This way of talking might sound a bit strange to people who are used to speaking English in other parts of the world, but it's completely normal in places like Canada, Scotland, and parts of England. It's just one of the many ways that people speak differently in different parts of the world!