An informal anarchist federation is like a group of friends who all like to play different games. Some of them like to play soccer, some like to play tag, and some like to play hide-and-seek. Even though they all like different games, they are still able to come together and have fun as a group.
In the same way, an informal anarchist federation is a group of people who all believe in the idea of anarchism, which means they don't like the idea of governments or hierarchies telling them what to do. Even though they might have different ideas about how to achieve anarchism or what it should look like, they are still able to come together and support each other's ideas and actions.
Just like the group of friends playing different games, the members of an informal anarchist federation might work on different projects or take different actions, but they all share the same goal of creating a world without governments or hierarchies. And even though they might not have an official leader or structure, they are still able to organize and work together towards this goal.