The red-green alliance is like when two friends who really like different things decide to work together on something. One friend really likes the color red, and the other friend really likes the color green. The first friend might wear a red shirt, and the second friend might wear a green shirt. Even though they like different colors, they both want to help each other out and work towards the same goal.
In the same way, the red-green alliance is formed when two different political groups, one who cares about social issues like healthcare and education (the red group), and the other who cares about environmental issues like protecting nature and preventing pollution (the green group), decide to work together towards a common goal. They might not agree on everything, but they both want to help each other out and make the world a better place.
For example, the red-green alliance may work together to pass laws that help improve access to healthcare while also protecting the environment. Or they may work together on policies that provide good jobs and a healthy environment for people to live in. It's all about finding common ground and working together towards a brighter future.