Migration diplomacy is when countries talk to each other about how their people move around the world. It's like when you invite your friend over to your house and your mom talks to their mom about what time they should be picked up.
But instead of just talking about one visit, they talk about lots of people going to different places for different reasons. Some people might be moving to another country to live there permanently or just temporarily. Some might be going to study or work there, and some might be escaping from danger in their home country.
The governments of different countries talk to each other so they can figure out how to help each other with all these people moving around. They want to make sure everything is safe and fair for everyone involved. That means setting rules for who can come in and how long they can stay, and making sure everyone has the right papers and is allowed to do what they want to do.
This is important because if countries don't talk to each other, people might get hurt or treated unfairly. But if they work together, everyone can have a chance to explore different places and build new lives.