A diplomatic passport is a special kind of passport that is given to people who work for the government and represent their country in other countries. It's like a special badge that helps them to travel more easily for work.
You know how when you go to another country, you have to show your regular passport to the people at the airport and sometimes they ask you lots of questions to make sure you're not a bad guy? Diplomats get to skip all those questions and they usually get to go through a special line that's faster because they are traveling for important government work. It's like they get to press an imaginary "I'm important" button that makes everything easier for them.
The reason why diplomats get this special passport is because they have very important jobs in other countries. For example, they might be an ambassador and need to meet with important people from the country they are visiting. Or, they might work in an embassy and have to help people from their own country who are visiting or living in the other country.
So, the diplomatic passport is like a super important passport that helps people who work for the government do their job more easily and safely when they are traveling to different countries.