Krestintern was a group of people who came together to help each other out a long time ago. They wanted to make the world a better place for everyone, especially people who worked hard for their money but didn't have a lot of power.
They did this by creating a way for workers from different countries to connect with each other and share ideas. They also helped with things like setting up unions and fighting for better working conditions.
Think of it like a club where everyone works together to help each other out and make things better for everyone. Krestintern was like that, but on a much bigger scale, with people from all over the world working together for a common goal.