Okay kiddo, let me explain the home port doctrine to you in simple terms. Imagine that you have a toy car that you really love and you always keep it in your room. One day, your friend comes over and he really likes your toy car too. He wants to play with it and take it home with him, but you tell him no because you always keep it in your room. Well, that's kind of like the home port doctrine.
The home port doctrine is a rule that says that ships and boats that belong to a country's navy have to stay in their home port most of the time. Just like you keep your toy car in your room, the navy keeps their ships at their home port.
But sometimes, the navy needs to use their ships for important things like patrols, training or rescuing people at sea. In that case, they can take their ships out of their home port, but they have to come back home when they're done.
It's kind of like how you can take your toy car out of your room to play with it, but you have to bring it back when you're done. The home port doctrine is just a way to make sure that everyone knows where the navy's ships are most of the time, and that they're ready to use them when they need to.