Okay kiddo, so the Johnson Doctrine is a fancy way of saying that the United States has a responsibility to protect other countries in the Americas (like Canada, Mexico, and all the countries in between) from any outside threats.
This idea came from President Lyndon B. Johnson back in the 1960s, when there were fears of communism spreading throughout the world. The US wanted to make sure that countries in the Americas didn't become communist too, so they said they would help defend them if needed.
Basically, it's like how you and your friends help each other out on the playground. If someone is picking on your friend, you step in to protect them. That's what the Johnson Doctrine is all about – the United States being a good friend to its neighbors and standing up for them if they need help.