The best way I find to think about it is a padlocked box.
The public key is a box with an open padlock on it. I can give it to anyone. If someone puts a message inside the box they can lock the padlock, but they don’t have the key to open it again.
I keep the key private. If someone sends me a locked box that has my padlock on it, only I have the key to open it and read the message.