Bucharest Nine is a group of nine countries in Europe that work together to help each other out. Think of it like when you and your friends make a team for a school project. These countries are Romania, Bulgaria, Poland, Czech Republic, Slovakia, Hungary, Lithuania, Latvia, and Estonia.
They all agreed to help each other out if one of them got in trouble or need support. They also talked about important things they want to do together, like protect their countries from bad guys, work together on energy and security things, and make sure they are prepared for any emergencies.
Just like how you and your friends have a plan to finish your project or help each other when things go wrong, these countries have made a plan to support each other, work together, and solve problems together.